Toxin perhaps in plant I generally cut down (7)
I believe the answer is:
antigen
'toxin perhaps' is the definition.
(toxin is a kind of antigen)
'plant i generally cut down' is the wordplay.
'cut down' says the answer is hidden in the clue (some letters cut away to reveal the answer).
'ANTIGEN' can be found hidden inside 'plant i generally'.
'in' acts as a link.
(Other definitions for antigen that I've seen before include "Substance inducing an immune response" , "Something inducing an immune response, neat gin (anag.)" , "Biological substance" , "Immunity-booster" , "toxin" .)