Despicable type's excessively large growth capped (9)
I believe the answer is:
toadstool
'growth capped' is the definition.
(fungus with a cap)
'despicable type's excessively large' is the wordplay.
'despicable type's' becomes 'toad's' (toad can mean a despicable person).
'excessively' becomes 'too' (synonyms).
'large' becomes 'L' (eg in clothes sizes).
'toads'+'too'+'l'='TOADSTOOL'
(Other definitions for toadstool that I've seen before include "Common fungus" , "jack-o'-lantern?" , "what's poisonous maybe" , "Plant akin to mushroom" , "Capped fungus" .)