Friend who cleans out the stables? (6)
I believe the answer is:
mucker
I believe this clue is a double definition.
'friend' is the first definition.
(affectionate term, as in 'my old mucker')
'who cleans out the stables?' is the second definition.
(someone mucking out a stables)
(Other definitions for mucker that I've seen before include "Friend (in a mess?)" , "Mate - shifter of rubbish" .)