Sweetheart having wild fling with rider (10)
I believe the answer is:
girlfriend
'sweetheart' is the definition.
(I've seen this before)
'wild fling with rider' is the wordplay.
'wild' indicates an anagram.
'with' means one lot of letters go next to another.
'fling' put after 'rider' is 'riderfling'.
'riderfling' anagrammed gives 'GIRLFRIEND'.
'having' acts as a link.
(Other definitions for girlfriend that I've seen before include "Female companion" , "Sweetheart" , "Female squeeze" , "date" .)