No prime consideration (12)
I believe the answer is:
'no prime consideration' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'prime consideration' is the wordplay.
'prime' becomes 'after' (I can't justify this - if you can you should believe this answer much more).
'consideration' becomes 'thought' (I've seen this before).
Can you help me to learn more?
(Other definitions for afterthought that I've seen before include "Re-evaluation" , "PS for instance" , "A reflection" , "Something added later" , "footnote" .)