Commander has to approve captain (4)
I believe the answer is:
cook
'captain' is the definition.
(captain james cook)
'commander has to approve' is the wordplay.
'commander' becomes 'co'.
'has' means one lot of letters go next to another.
'to approve' becomes 'ok' ('ok' can be a synonym of 'approve').
'co'+'ok'='COOK'
(Other definitions for cook that I've seen before include "Heat food" , "Captain of HMS Endeavour" , "New England Test captain" , "Kitchen worker" , "Prepare a hot meal" .)