Very young communist as a fellow serviceman (7,2,4)
I believe the answer is:
comrade in arms
'fellow serviceman' is the definition.
(I've seen this before)
'very young communist as a' is the wordplay.
I cannot really understand how this works, but
'as' could be 'in' (I've seen this in other clues) and 'in' is found in the answer.
'a' is within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for comrade in arms that I've seen before include "Fellow serviceman" , "Fellow soldier" .)