Enormous old boy feasted regularly (5)
I believe the answer is:
obese
'enormous' is the definition.
(I've seen this before)
'old boy feasted regularly' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'feasted' are 'ese'.
'ob'+'ese'='OBESE'
(Other definitions for obese that I've seen before include "Pathologically fat" , "having generous corporation" , "Needing to lose weight" , "with a pot?" , "Enormous" .)