Government committee's firm support (5)
I believe the answer is:
cobra
'government committee's' is the definition.
(UK government crisis committee)
'firm support' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'support' becomes 'bra' (a support for the breasts).
'co'+'bra'='COBRA'
(Other definitions for cobra that I've seen before include "government emergency committee?" , "a king, for example" , "venomous type" , "Type of snake" , "hamadryad" .)