Firm underwear - it can hurt you badly (5)
I believe the answer is:
cobra
'it can hurt you badly' is the definition.
The definition suggests a singular noun which matches the answer.
'firm underwear' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'underwear' becomes 'bra' (bra is a type of undergarment).
'co'+'bra'='COBRA'
Can you help me to learn more?
(Other definitions for cobra that I've seen before include "one leading charmed life?" , "government emergency committee?" , "Reptile (found in Cabinet Office Briefing Room A?)" , "Venomous snake, may be king" , "Highly venomous hooded reptile" .)