Explorer heading westward, company leaves (7)
I believe the answer is:
tobacco
'leaves' is the definition.
(I've seen this before)
'explorer heading westward company' is the wordplay.
'explorer' becomes 'cabot' (I've seen this before).
'heading westward' is a reversal indicator.
'company' becomes 'co' (abbreviation).
'cabot' back-to-front is 'tobac'.
'tobac'+'co'='TOBACCO'
(Other definitions for tobacco that I've seen before include "Weed dangerous to health" , "Dried leaves for smoking" , "20 [CASTRO] 's favourite leaves" , "Addictive leaf" , "Put that in your pipe and smoke it!" .)