Food for Pegasus, not Gus (4)
I believe the answer is:
peas
'food' is the definition.
(I've seen this in another clue)
'pegasus not gus' is the wordplay.
'not' indicates named letters should be taken away.
'pegasus' with 'gus' removed is 'PEAS'.
'for' acts as a link.
(Other definitions for peas that I've seen before include "Podded vegetables" , "pulse" , "Pulses" , "Seeded vegetables" , "Leguminous plants" .)