Got worse, being wrinkled at end of month (8)
I believe the answer is:
declined
'got worse' is the definition.
('decline' can be a synonym of 'get worse')
'wrinkled at end of month' is the wordplay.
'wrinkled' becomes 'lined' (I've seen this before).
'at end of' means one lot of letters go next to another.
'month' becomes 'dec' (Dec is an example).
'lined' after 'dec' is 'DECLINED'.
'being' is the link.
(Other definitions for declined that I've seen before include "Refused, rejected" , "Refused or weakened" , "Went through cases" , "Refused; waned" , "Refused or became weaker" .)