One taking pains to enjoy Russian resort: nuts about it (9)
I believe the answer is:
masochist
'one taking pains to enjoy' is the definition.
(someone who enjoys pain)
'russian resort nuts about it' is the wordplay.
'russian resort' becomes 'Sochi' (resort city in Russia).
'nuts' becomes 'mast' (mast is the nuts of forest trees).
'about it' is an insertion indicator.
'sochi' put inside 'mast' is 'MASOCHIST'.
(Other definitions for masochist that I've seen before include "One who likes pain" , "one enjoying humiliation" , "deviant"" .)