Gibson, say, has a feast (4)
I believe the answer is:
meal
'feast' is the definition.
(thesaurus)
'gibson say has a' is the wordplay.
'gibson say' becomes 'mel' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'has' indicates putting letters inside.
'mel' enclosing 'a' is 'MEAL'.
Can you help me to learn more?
(Other definitions for meal that I've seen before include "Eg, lunch" , "Edible part of grain or pulse" , "Food for male" , "Eg, breakfast" , "Something to eat" .)