One advancing with month remaining (5)
I believe the answer is:
mover
'one advancing' is the definition.
(a person who moves)
'with month remaining' is the wordplay.
'with' means one lot of letters go next to another.
'month' becomes 'm' (standard abbreviation).
'remaining' becomes 'over' (as in 'we ate them all but had one over').
'm' put next to 'over' is 'MOVER'.
(Other definitions for mover that I've seen before include "Formal proposer" , "someone who brings forward proposal" , "Instigator, organiser" , "Dynamic type" , "One who is in motion - may also be a shaker" .)