Grub had after terrible AGM (6)
I believe the answer is:
maggot
'grub' is the definition.
(maggot is a kind of grub)
'had after terrible agm' is the wordplay.
'had' becomes 'got' ('get' can be a synonym of 'have').
'after' is a charade indicator (letters next to each other).
'terrible' is an anagram indicator.
'agm' anagrammed gives 'mag'.
'got' after 'mag' is 'MAGGOT'.
(Other definitions for maggot that I've seen before include "Larva often found in decaying matter" , "Legless larva" , "immature flyer" , "Lava of a fly" , "Fly larva" .)