Informer ultimately done for speed (4)
I believe the answer is:
rate
'for speed' is the definition.
(both can mean pace)
'informer ultimately done' is the wordplay.
'informer' becomes 'rat' (synonyms).
'ultimately' suggests the final letters.
The last letter of 'done' is 'e'.
'rat'+'e'='RATE'
(Other definitions for rate that I've seen before include "See 11" , "Value; charge" , "Price - class - speed" , "Assign a value to, appraise" , "See 12" .)