He's slow, sounds dull and fat! (7)
I believe the answer is:
dullard
'he's slow sounds' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'dull and fat' is the wordplay.
'and' is a charade indicator (letters next to each other).
'fat' becomes 'ard' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'dull'+'ard'='DULLARD'
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" .)