Rodent ultimately done for speed (4)
I believe the answer is:
rate
'speed' is the definition.
(rate is a kind of speed)
'rodent ultimately done' is the wordplay.
'rodent' becomes 'rat' (rat is a kind of rodent).
'ultimately' says to take the final letters.
The last letter of 'done' is 'e'.
'rat'+'e'='RATE'
'for' acts as a link.
(Other definitions for rate that I've seen before include "The price paid" , "See 11" , "Estimate value of - speed" , "See 16" , "Merit; speed" .)