Billy hiding water from a fowl (5)
I believe the answer is:
capon
'a fowl' is the definition.
(I know that capon is a type of Gallus gallus)
'billy hiding water' is the wordplay.
'billy' becomes 'can' (I've seen this before).
'hiding' means one lot of letters goes inside another.
'water' becomes 'po' (Po is an example).
'can' enclosing 'po' is 'CAPON'.
'from' acts as a link.
(Other definitions for capon that I've seen before include "Castrated rooster fattened for eating" , "Emasculated fowl" , "Castrated fowl, for eating" , "poultry?" , "one fattened specially" .)