Joking concerning certain aspects about a credit note (9)
I believe the answer is:
facetious
'joking' is the definition.
(thesaurus)
'aspects about a credit note' is the wordplay.
'aspects' becomes 'facets' ('facet' can be a synonym of 'aspect').
'about' indicates putting letters inside.
'a credit note' becomes 'iou' (I can't explain this - if you can you should believe this answer much more).
'facets' enclosing 'iou' is 'FACETIOUS'.
'concerning certain' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for facetious that I've seen before include "Waggish" , "Intended to be amusing or witty" , "Using inapt humour" , "Jocular; frivolous" , "Flippantly humorous" .)