Pitman restraining duke's bodyguard (6)
I believe the answer is:
minder
'bodyguard' is the definition.
(I've seen this before)
'pitman restraining duke's' is the wordplay.
'pitman' becomes 'miner' (I've seen this before).
'restraining' indicates putting letters inside.
'duke' becomes 'd' (abbreviation).
'miner' enclosing 'd' is 'MINDER'.
(Other definitions for minder that I've seen before include "One looking after" , "baby sitter?" , "One who objects" , "Bodyguard (colloq.)" , "One who looks after (someone or something)" .)