Strike back at a serviceman (4)
I believe the answer is:
para
'serviceman' is the definition.
(para is a kind of serviceman)
'strike back at a' is the wordplay.
'strike' becomes 'rap' (synonyms).
'back' is a reversal indicator.
'at' says to put letters next to each other.
'rap' written backwards gives 'par'.
'par'+'a'='PARA'
(Other definitions for para that I've seen before include "A block of text; an airborne soldier (abbrs.)" , "Soldier who's dropped in? (abbr)" , "Airdropped soldier" , "High-jumper" , "Soldier (shortened)" .)