So Long, half cut single (4)
I believe the answer is:
solo
'single' is the definition.
(I've seen this before)
'so long half cut' is the wordplay.
'half cut' suggests halving the letters.
'long' halved is 'lo'.
'so'+'lo'='SOLO'
(Other definitions for solo that I've seen before include "Lone" , "Piece for single performer" , "Type of whist" , "Card game; for one" , "Done by one person" .)