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 really see how this works, but
'slow' could be 'dull' (synonyms) and 'dull' is found within the answer.
The remaining letters 'ard' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'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" .)