Crowd retreats, heading away from Mexican city plant (9)
I believe the answer is:
marijuana
'plant' is the definition.
(I know that marijuana is a plant)
'crowd retreats heading away from mexican city' is the wordplay.
'crowd' becomes 'ram' (I've seen this before).
'retreats' says the letters should be written backwards (I've seen 'retreat' mean this).
'heading away from' means to remove the first letter.
'mexican city' becomes 'tijuana' (I've seen this before).
'ram' backwards is 'mar'.
'tijuana' with its first letter taken off is 'ijuana'.
'mar'+'ijuana'='MARIJUANA'
(Other definitions for marijuana that I've seen before include "Pot" , "3 Down" , "Recreational drug" , "grass" , "Cannabis" .)