Lady exposes Mellors's rear, holding on (8)
I believe the answer is:
baroness
'lady' is the definition.
(I've seen this before)
'exposes mellors's rear holding on' is the wordplay.
'exposes' becomes 'bares' (baring is a kind of exposing**).
'rear' indicates one should take the final letters (the back of the word).
'holding' indicates putting letters inside.
The last letter of 'mellors' is 's'.
'bares'+'s'='baress'
'baress' enclosing 'on' is 'BARONESS'.
(Other definitions for baroness that I've seen before include "Noble" , "Lady peer" , "Noblewoman" , "Lady Thatcher?" , "Thatcher possibly" .)