Smarter without one chat (6)
I believe the answer is:
natter
'chat' is the definition.
(synonyms)
'smarter without one' is the wordplay.
'smarter' becomes 'nattier' (I've seen this before).
'without' indicates named letters should be taken away.
'one' becomes 'i' (Roman numeral).
'nattier' with 'i' removed is 'NATTER'.
(Other definitions for natter that I've seen before include "Chatter (colloq.)" , "Idle chat" , "Talk casually" , "Chew the fat, shoot the breeze" , "Talk inconsequentially" .)