Sinuous creature's firm support (5)
I believe the answer is:
cobra
'sinuous creature's' is the definition.
(I've seen this before)
'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 "Sinuous creature" , "Hooded snake - AC model of old sports car" , "Something with a hood" , "venomous type" , "Venomous hooded snake" .)