Male domestic fowl (7)
'male domestic fowl' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rooster that I've seen before include "Adult male chicken" , "Adult male domestic fowl" , "Farmyard bird" , "Domestic cock" , "male with comb" .)
(Other definitions for cock that I've seen before include "Component of gun" , "Pull back a gun's hammer in readiness to shoot" , "Set upright" , "Male lobster; valve" , "Small heap of hay" .)