Protection against kids worn by a big chicken (5)
I believe the answer is:
capon
'a big chicken' is the definition.
'protection against kids worn' is the wordplay.
'protection against kids' becomes 'cap' (type of contraceptive).
'worn' becomes 'on'.
'cap'+'on'='CAPON'
'by' acts as a link.
(Other definitions for capon that I've seen before include "Male chicken for eating" , "Castrated cock fowl" , "one fattened specially" , "Castrated fowl, for eating" , "poultry?" .)