Goat in cooking pot (5)
I believe the answer is:
I believe this clue is a double definition.
'goat' is the first definition.
(billy is a kind of goat)
'cooking pot' is the second definition.
(I've seen this before)
'in' is the link.
(Other definitions for billy that I've seen before include "Camper's pot" , "Light metal cooking pot used for camping" , "Goat; camper's kettle" , "Camp can" , "Metal camping can for tea" .)