Expert during square-bashing disheartened military recruit (5)
I believe the answer is:
sprog
'recruit' is the definition.
(I know that sprog is a type of recruit)
'expert during square-bashing disheartened military' is the wordplay.
I cannot really see how this works, but
'expert' could be 'pro' (short for professional) and 'pro' is found within the answer.
'square' could be 's' (abbreviation) and 's' is found within the answer.
A single letter 'g' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sprog that I've seen before include "Infant" , "Baby (slang)" , "Kid" , "Child; recruit (mil. slang)" , "little one" .)