Mostly produce a favourable reaction on a Spanish flower producer (10)
I believe the answer is:
impresario
'producer' is the definition.
(I know that impresario is a type of producer)
'mostly produce a favourable reaction on a spanish flower' is the wordplay.
'mostly' means to remove the last letter.
'produce a favourable reaction' becomes 'impress' (as in impressing someone).
'on' means one lot of letters go next to another.
'spanish flower' becomes 'rio' ('river' in Spanish - a river flows).
'impress' with its last letter taken away is 'impres'.
'impres'+'a'+'rio'='IMPRESARIO'
(Other definitions for impresario that I've seen before include "Theatrical manager" , "opera manager" , "Arranger of concerts, etc" , "Showman" , "As company manager" .)