Powerful man, head, restraining sailor (5)
I believe the answer is:
nabob
'powerful man' is the definition.
(I've seen this before)
'head restraining sailor' is the wordplay.
'head' becomes 'nob' (I've seen this in other clues).
'restraining' indicates putting letters inside.
'sailor' becomes 'ab' (abbreviation for able seaman).
'nob' enclosing 'ab' is 'NABOB'.
(Other definitions for nabob that I've seen before include "Magnate; formerly one with Eastern wealth" , "Devotional painting" , "One divining (water)" , "Person of great wealth" , "Mogul governor; rich man" .)