Silent songbird catching a cold (5)
I believe the answer is:
tacit
'silent' is the definition.
(I know that silent can be written as tacit)
'songbird catching a cold' is the wordplay.
'songbird' becomes 'tit' (tit is a type of songbird).
'catching' means one lot of letters goes inside another.
'cold' becomes 'c' (eg on taps).
'a'+'c'='ac'
'tit' going around 'ac' is 'TACIT'.
(Other definitions for tacit that I've seen before include "Implied without being said" , "Aircraft engine casing" , "Implied or understood, not put into words" , "Understood, implicit" , "Unvoiced" .)