Traitor ultimately done for speed (4)
I believe the answer is:
rate
'speed' is the definition.
(the speed at which something happens)
'traitor ultimately done' is the wordplay.
'traitor' becomes 'rat' (I've seen this before).
'ultimately' indicates one should take the final letters.
The final letter of 'done' is 'e'.
'rat'+'e'='RATE'
'for' acts as a link.
(Other definitions for rate that I've seen before include "Relative speed (of progress)" , "Estimate or deserve" , "Amount measures against another" , "Measure as of value or speed" , "Assess; be worthy of" .)