Free to finish unsettled detail (5,3)
I believe the answer is:
loose end
'unsettled detail' is the definition.
(I've seen this in another clue)
'free to finish' is the wordplay.
'free' becomes 'loose' (synonyms).
'to finish' becomes 'end' (synonyms).
'loose'+'end'='LOOSE END'
(Other definitions for loose end that I've seen before include "One's not yet decided" , "Detail not yet settled" , "'A detail not settled or explained (5,3)'" , "Unsettled matter" , "should have been tied?" .)