Mine contains half a loaf for the captain (5)
I believe the answer is:
pilot
'captain' is the definition.
(thesaurus)
'mine contains half a loaf' is the wordplay.
'mine' becomes 'pit' (down the pit).
'contains' means one lot of letters goes inside another.
'half a' suggests halving the letters.
'loaf' halved is 'lo'.
'pit' placed around 'lo' is 'PILOT'.
'for the' acts as a link.
(Other definitions for pilot that I've seen before include "Steer (a ship or plane)" , "Aviator" , "Name of Mr Rochester's dog" , "Navigator" , "Trial broadcast" .)