Foolish proof that not all adults get fat (7)
I believe the answer is:
dullard
'fat' is the definition.
(thesaurus)
'foolish proof that not all adults' is the wordplay.
I cannot quite see how this works, but
an anagram of 'all' is 'lla' which is present in the answer.
This explanation may well be incorrect...
'get' is the link.
Can you help me to learn more?
(Other definitions for dullard that I've seen before include "idiot" , "Slow or stupid person" , "Boring type" , "simpleton" , "Stupid fellow" .)