Bird covering bill becomes silent (5)
I believe the answer is:
tacit
'silent' is the definition.
(I know that silent can be written as tacit)
'bird covering bill' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'covering' indicates putting letters inside.
'bill' becomes 'ac' (abbreviation for account).
'tit' enclosing 'ac' is 'TACIT'.
'becomes' is the link.
(Other definitions for tacit that I've seen before include "Silent but implied" , "Attic? (anag)" , "Implicitly understood" , "Implied -- inferred" , "Implied but not directly expressed" .)