Finish fast in true style, say (5)
I believe the answer is:
utter
'say' is the definition.
(to utter something is to speak or say it)
'finish fast in true style' is the wordplay.
'finish' says to take the final letters.
'in' is an insertion indicator.
'style' indicates an anagram (style can mean to shape or design).
The last letter of 'fast' is 't'.
'true' with letters rearranged gives 'uter'.
't' placed within 'uter' is 'UTTER'.
(Other definitions for utter that I've seen before include "in toto" , "Absolute - state" , "Unqualified - unadulterated" , "Express; complete" , "Put into words" .)