He's slow, sounds dull and fat! (7)
I believe the answer is:
dullard
'fat' is the definition.
(thesaurus)
'he's slow sounds dull' is the wordplay.
I cannot quite understand how this works, but
'slow' could be 'dull' (synonyms) and 'dull' is located in the answer.
The remaining letters 'ard' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'and' is the link.
Can you help me to learn more?
(Other definitions for dullard that I've seen before include "simpleton" , "Slow or stupid person" , "Boring type" , "Dope" , "Stupid fellow" .)