Silent, antagonistic son joins army (7)
I believe the answer is:
enlists
'joins army' is the definition.
'silent antagonistic son' is the wordplay.
'antagonistic' is an anagram indicator.
'son' becomes 's' ().
'silent' is an anagram of 'enlist'.
'enlist'+'s'='ENLISTS'
(Other definitions for enlists that I've seen before include "Recruits" , "Listens (anag.)" , "Registers" , "volunteers" , "Signs up" .)