Back massage, skirting large unclear spot (4)
I believe the answer is:
blur
'unclear spot' is the definition.
(an unclear or blurred area)
'back massage skirting large' is the wordplay.
'back' is a reversal indicator.
'massage' becomes 'rub' (I've seen this before).
'skirting' indicates putting letters inside.
'large' becomes 'l'.
'rub' enclosing 'l' is 'rulb'.
'rulb' backwards is 'BLUR'.
(Other definitions for blur that I've seen before include "Britpop band - indistinct vision" , "Indistinct object - smudge" , "Smear, make indistinct" , "Unclear image, of a pop band" , "Befog" .)