Silent about having to join up (6)
I believe the answer is:
enlist
'to join up' is the definition.
(I've seen this before)
'silent about' is the wordplay.
'about' indicates anagramming the letters.
'silent' anagrammed gives 'ENLIST'.
'having' is the link.
(Other definitions for enlist that I've seen before include "Draft" , "Join the army" , "Enrol - recruit" , "Take the Queen's shilling" , "Join the military" .)