Silent bird without bill (5)
I believe the answer is:
tacit
'silent' is the definition.
(I know that silent can be written as tacit)
'bird without bill' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'bill' becomes 'ac' (short for account).
'tit' going around 'ac' is 'TACIT'.
(Other definitions for tacit that I've seen before include "Implicitly understood" , "Implied without being put into words" , "Implied -- inferred" , "Implied, not spoken" , "Understood without being put into words" .)