Total idiot ultimately, partner (5)
I believe the answer is:
tally
'total' is the definition.
('tally' can be a synonym of 'total')
'idiot ultimately partner' is the wordplay.
'ultimately' indicates one should take the final letters.
'partner' becomes 'ally' (I've seen this in other clues).
The final letter of 'idiot' is 't'.
't'+'ally'='TALLY'
(Other definitions for tally that I've seen before include "Agree, add up" , "Total score" , "Score; correspond" , "highly?" , "Count, add up" .)