Rap elbow nastily, a bit unwell (5,3)
I believe the answer is:
below par
'a bit unwell' is the definition.
'rap elbow nastily' is the wordplay.
'nastily' is an anagram indicator.
'rap'+'elbow'='rapelbow'
'rapelbow' anagrammed gives 'BELOW PAR'.
(Other definitions for below par that I've seen before include "Not in full health" , "Lower than the average, as in golf score ( 5, 3)" , "Worse than expected" , "Not up to usual standard, like golfer" , "Worse than usual" .)