Having considered the matter offers footnote (12)
I believe the answer is:
afterthought
'footnote' is the definition.
(I've seen this before)
'having considered the matter' is the wordplay.
I cannot really understand how this works, but
'considered' could be 'thought' (I've seen this before) and 'thought' is found within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
'matter' could be 'fe' (Fe is an example) and 'fe' is found in the remaining letters.
The remaining letters 'ar' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'offers' is the link.
Can you help me to learn more?
(Other definitions for afterthought that I've seen before include "Later addition" , "not an original idea" , "Re-evaluation" , "Something added later" , "PS for instance" .)