Potato remains with first bit of mustard (4)
I believe the answer is:
mash
'potato' is the definition.
(mashed potato)
'remains with first bit of mustard' is the wordplay.
'remains' becomes 'ash' (remains of a fire).
'with' is a charade indicator (letters next to each other).
'first bit of mustard' becomes 'm' (1st letter of 'mustard').
'ash' put after 'm' is 'MASH'.
(Other definitions for mash that I've seen before include "food for animals" , "Pured potato" , "It goes well with sausages" , "Sausages' traditional partner" , "Food for horses" .)