Rotter finally done for speed (4)
I believe the answer is:
rate
'speed' is the definition.
(the speed at which something happens)
'rotter finally done' is the wordplay.
'rotter' becomes 'rat' (synonyms).
'finally' suggests the final letters.
The last letter of 'done' is 'e'.
'rat'+'e'='RATE'
'for' is the link.
(Other definitions for rate that I've seen before include "Value; charge" , "The speed of movement" , "The number of times something happens in a given period" , "Relative speed (of progress)" , "Harshly rebuke" .)