Outstanding first bit of tart -- a piece for two (4)
I believe the answer is:
duet
'a piece for two' is the definition.
(I've seen this before)
'outstanding first bit of tart' is the wordplay.
'outstanding' becomes 'due' (I've seen this before).
'first bit of' says to take the initial letters.
The initial letter of 'tart' is 't'.
'due'+'t'='DUET'
(Other definitions for duet that I've seen before include "Work for two performers" , "Composition for two performers" , "Piece for brace" , "Performance by two" , "Work for two players" .)