Sign erected in an English plant (7)
I believe the answer is:
anemone
'plant' is the definition.
(anemone is a kind of plant)
'sign erected in an english' is the wordplay.
'sign' becomes 'omen' (omen is a kind of sign).
'erected' is a reversal indicator (in a down clue, letters go up).
'in' is an insertion indicator.
'english' becomes 'e' (abbreviation).
'omen' reversed gives 'nemo'.
'nemo' inserted inside 'an' is 'anemon'.
'anemon'+'e'='ANEMONE'
(Other definitions for anemone that I've seen before include "Plant with bright flowers" , "Tentacled sea dweller, flower" , "bloom" , "bloomer" , "Flowering plant - mean one (anag)" .)