Silly person, ultimately without intelligence (4)
I believe the answer is:
twit
'silly person' is the definition.
(I've seen this before)
'ultimately without intelligence' is the wordplay.
'ultimately' suggests the final letters.
'intelligence' becomes 'wit' (wit is a kind of intelligence).
The last letter of 'without' is 't'.
't'+'wit'='TWIT'
(Other definitions for twit that I've seen before include "Nincompoop" , "Silly person; tease" , "dunce" , "Blockhead" , "Silly ass" .)