Explorer returns and officer leaves (7)
I believe the answer is:
tobacco
'leaves' is the definition.
(tobacco leaves)
'explorer returns and officer' is the wordplay.
'explorer' becomes 'cabot' (I've seen this before).
'returns' is a reversal indicator.
'and' is a charade indicator (letters next to each other).
'officer' becomes 'co' (Commanding Officer abbreviation).
'cabot' written backwards gives 'tobac'.
'tobac'+'co'='TOBACCO'
(Other definitions for tobacco that I've seen before include "Cigarette filling" , "(Treated leaves of) American plant" , "Popular narcotic" , "20 [CASTRO] 's favourite leaves" , "Smoker's leaf" .)