Love letter, unfinished, turning up (3)
I believe the answer is:
nil
'love' is the definition.
(I've seen this before)
'letter unfinished turning up' is the wordplay.
'letter' becomes 'line' (to drop someone a line is to write them a letter).
'unfinished' means to remove the last letter.
'turning up' says the letters should be written backwards.
'line' with its final letter taken away is 'lin'.
'lin' backwards is 'NIL'.
(Other definitions for nil that I've seen before include "Zero, no score" , "Zero, zilch" , "There's nothing left" , "You get nothing" , "Nothing - in Latin" .)