An officer has, below him, many a sailor (7)
I believe the answer is:
matelot
'sailor' is the definition.
(I've seen this before)
'an officer has below' is the wordplay.
'an officer' becomes 'mate' (mate is a kind of officer).
'has below' becomes 'lot' (I can't justify this - if you can you should believe this answer much more).
'mate'+'lot'='MATELOT'
'him many a' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for matelot that I've seen before include "A lot met (anag) - sailor" , "One sailor" , "Rating" , "seafarer" , "Sailor (slang from French)" .)