The bit where they split up? (4)
I believe the answer is:
part
'split up?' is the definition.
(part can mean to split or separate)
'the bit where they' is the wordplay.
I cannot really understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
The remaining letters 'par' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for part that I've seen before include "Section of a whole" , "Piece, segment" , "Some - theatrical role" , "Member" , "Element" .)