Bird without bill is silent (5)
I believe the answer is:
tacit
'silent' is the definition.
('tacit' can be a synonym of 'silent')
'bird without bill' is the wordplay.
'bird' becomes 'tit' (I've seen this before**).
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'bill' becomes 'ac'.
'tit' enclosing 'ac' is 'TACIT'.
'is' is the link.
(Other definitions for tacit that I've seen before include "Implicit, unspoken" , "Understood, not spoken or played" , "Unspoken but implied" , "Silent - understood" , "Attic? (anag)" .)