Fatter, out of shape, nevertheless tucking in? Here's further idea (12)
I believe the answer is:
afterthought
'further idea' is the definition.
(an additional idea)
'fatter out of shape nevertheless tucking in?' is the wordplay.
'out of shape' is an anagram indicator.
'nevertheless' becomes 'though' (I've seen this before).
'tucking in?' indicates putting letters inside.
'fatter' with letters rearranged gives 'aftert'.
'aftert' enclosing 'though' is 'AFTERTHOUGHT'.
'here's' is the link.
(Other definitions for afterthought that I've seen before include "footnote" , "not an original idea" , "Rider" , "PS for instance" , "Later addition" .)