Swell chaps ending in debt after a month (7)
I believe the answer is:
augment
'swell' is the definition.
(thesaurus)
'chaps ending in debt after a month' is the wordplay.
'chaps' becomes 'men' (I've seen this before).
'ending in' says to take the final letters.
'after' means one lot of letters go next to another.
'a month' becomes 'aug' (Aug is an example).
The last letter of 'debt' is 't'.
'men'+'t'='ment'
'ment' put after 'aug' is 'AUGMENT'.
(Other definitions for augment that I've seen before include "Grow - intensify" , "boost" , "get longer" , "Enlarge or increase" , "Swell" .)