Swell chaps replacing us in a month (7)
I believe the answer is:
augment
'swell' is the definition.
(I've seen this before)
'chaps replacing us in a month' is the wordplay.
I cannot really understand how this works, but
'chaps' could be 'men' (I've seen this before) and 'men' is found in the answer.
'a' is found in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for augment that I've seen before include "Swell" , "Increase number of" , "get longer" , "Supplement" , "Grow - intensify" .)