Rock partner's body (5)
I believe the answer is:
torso
'body' is the definition.
(a person's middle)
'rock partner's' is the wordplay.
I cannot really understand how this works, but
'rock' could be 'tor' (a tor is a rocky peak) and 'tor' is located in the answer.
'partner' could be 'so' (abbreviation for significant other) and 'so' is located in the answer.
No letters remain.
This explanation may well be incorrect...
(Other definitions for torso that I've seen before include "Poorly-dressed child" , "Roots (anag)" , "Body without head, neck and limbs" , "Roost (anag)" , "Main part of human body" .)