Girl I love breaks arm in retreat (5)
I believe the answer is:
moira
'girl' is the definition.
'i love breaks arm in retreat' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'breaks' means one lot of letters goes inside another.
'in retreat' says the letters should be written backwards.
'i'+'o'='io'
'io' put within 'arm' is 'ariom'.
'ariom' reversed gives 'MOIRA'.
(Other definitions for moira that I've seen before include "Any of the three Greek Fates" , "Shearer perhaps" , "-- Shearer, ballet dancer" , "Shearer, possibly" , "Girl" .)