Clampdown regularly culled fowl (5)
I believe the answer is:
capon
'fowl' is the definition.
(I know that capon is a type of Gallus gallus)
'clampdown regularly culled' is the wordplay.
'regularly culled' means one should take alternating letters (letters removed at regular intervals).
The alternating letters of 'clampdown' are 'CAPON'.
(Other definitions for capon that I've seen before include "Castrated bird" , "Bird fattened for eating" , "Male chicken for eating" , "poultry?" , "Male fowl fattened for eating" .)