This clue type uses the first or last (yes, I know) letters of words to help make up part or all of the answer word. To spot an initialism look for indicators that suggest the first or last letter or head or tail of a piece of fodder is to be used.
Here are some examples of first and last letter indicators:
To indicate S the setter may write:
head of school ...
.... the head of school is S - the first letter (or head) of the word SCHOOL.
At another time, the setter may wish to indicate the letter L to help construct the answer word, and so she can use the same fodder SCHOOL with a different indicator:
Last in School ...
... the last in school is L - the last letter in the word SCHOOL
Any words that fairly indicate the first or last letters of a word can be used as first or last letter indicators. Here are a few more examples.
first letter indicator - FIRST OF/FRONT TO/ORIGINALLY
last letter indicator - LAST OF/END IN/ULTIMATELY
Finally, as with many indicators, the type of first or last letter indicator used will be dependent on whether the clue is across or down. Down clues will use first and last letter indicators that are either specific to a vertical positioning (BOTTOM OF) or that are non-horizontal specific (LAST OF) and vice-versa for horizontal clues.
NOTE: With this clue type you will normally not have to find a synonym before taking the indicated letters.
Whilst initialisms are more often used to indicate a single letter, here are some examples where they form the whole or part of the answer word.
Network heads to make erotic series happen (4)
network - definition
heads to - initial letter indicator
make - m
erotic - e
series - s
happen - h
A reverse initialism:
After a set-back, heads of school display no exam results (4)
after a set-back - reversal indicator
heads of - initial letter indicator
school - s
display - d
no - n
exam - e
results - definition
(E)(N)(D)(S) < = ENDS
Part initialism (using end letters)
Look to credit ultimately for money (4)
look = lo (a poetic form of look)
to - o (the ultimate letter of TO)
credit - t (the ultimate letter of CREDIT)
ultimately - last letter indicator
money - definition