This girl exercises with a heartless rival (5)
I believe the answer is:
pearl
'this girl' is the definition.
'exercises with a heartless rival' is the wordplay.
'exercises' becomes 'pe' (**).
'with' means one lot of letters go next to another.
'heartless' suggests removing the centre.
'rival' with its middle taken out is 'rl'.
'pe'+'a'+'rl'='PEARL'
(Other definitions for pearl that I've seen before include "Gem; an attacked Harbor" , "Gem from a bivalve" , "See 12" , "Gem from the sea" , "Gem from oyster" .)