Fight in holiday resort, reportedly (4)
I believe the answer is:
'fight' is the definition.
(sparring is a kind of fighting)
'holiday resort reportedly' is the wordplay.
'holiday resort' becomes 'spa' (spa is a kind of holiday resort).
'reportedly' becomes 'r' (this could be a standard abbreviation of which I'm unaware).
'in' is the link.
(Other definitions for spar that I've seen before include "Pole used to support sails or rigging" , "Act as boxer" , "Raps with wooden beam" , "Shadow-box" , "Argue" .)