Nothing in watering hole for pig (4)
I believe the answer is:
boar
'pig' is the definition.
(I've seen this in other clues)
'nothing in watering hole' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'in' means one lot of letters goes inside another.
'watering hole' becomes 'bar' (I've seen this before).
'o' inserted within 'bar' is 'BOAR'.
'for' acts as a link.
(Other definitions for boar that I've seen before include "Uncastrated male hog" , "A male pig, sounds tedious" , "Wild swine" , "Sow's mate" , "Resting place for birds or bats" .)