What set back ape? Blocked by large plant (7)
I believe the answer is:
hemlock
'plant' is the definition.
(hemlock is a kind of plant)
'what set back ape? blocked by large' is the wordplay.
'what' becomes 'eh' (I've seen this in other clues).
'set back' is a reversal indicator.
'ape?' becomes 'mock' (aping is a kind of mocking**).
'blocked by' indicates putting letters inside (I've seen this in other clues).
'large' becomes 'l'.
'eh' in reverse letter order is 'he'.
'mock' enclosing 'l' is 'mlock'.
'he'+'mlock'='HEMLOCK'
(Other definitions for hemlock that I've seen before include "Poisonous plant" , "Socrates's poison" , "Poisonous drug" , "Highly poisonous relative of parsley" , "Poisonous weed" .)