Illness caused by gap in credit - source of anxiety (7)
I believe the answer is:
cholera
'illness' is the definition.
(cholera is a kind of illness)
'gap in credit source of anxiety' is the wordplay.
'gap' becomes 'hole' (hole is a kind of gap**).
'in' means one lot of letters goes inside another.
'credit' becomes 'cr'.
'source of' says to take the initial letters.
The first letter of 'anxiety' is 'a'.
'hole' going within 'cr' is 'choler'.
'choler'+'a'='CHOLERA'
'caused by' acts as a link.
(Other definitions for cholera that I've seen before include "Intestinal infection caused by contaminated water or food" , "Contagious disease" , "Disease caught from contaminated water" , "Infectious disease causing severe bowel upset" , "Disease contracted from infected water" .)