School getting parent's letter (5)
I believe the answer is:
gamma
'letter' is the definition.
(gamma is a kind of letter of the alphabet)
'school getting parent's' is the wordplay.
'school' becomes 'gam' (I've seen this in other clues).
'getting' means one lot of letters go next to another.
'parent' becomes 'ma' (ma can mean mum or mother).
'gam'+'ma'='GAMMA'
(Other definitions for gamma that I've seen before include "Foreign character" , "Greek letter - type of rays" , "Third letter of the Greek alphabet" , "Greek character" , "third highest grade" .)