Grand American brought back mysterious crop from tropics (5,4)
I believe the answer is:
sugar cane
'crop from tropics' is the definition.
(crop grown in the tropics)
'grand american brought back mysterious' is the wordplay.
'grand' becomes 'G' (G is short for ground as in a thousand).
'american' becomes 'us' (abbreviation for United States).
'brought back' is a reversal indicator.
'mysterious' becomes 'arcane' (I've seen this before).
'g'+'us'='gus'
'gus' reversed gives 'sug'.
'sug'+'arcane'='SUGAR-CANE'
(Other definitions for sugar cane that I've seen before include "Grass" , "Source of sweet stuff" , "What makes a spirit" , "Source of sweetener" , "supergrass?" .)