Job to finish fast by request (4)
I believe the answer is:
task
'job' is the definition.
(I know that job can be written as task)
'finish fast by request' is the wordplay.
'finish' suggests the final letters.
'by' means one lot of letters go next to another.
'request' becomes 'ask' (to ask is to request something).
The last letter of 'fast' is 't'.
't'+'ask'='TASK'
'to' acts as a link.
(Other definitions for task that I've seen before include "Difficult piece of work" , "Sort of force" , "Job to be done" , "Burden with work" , "Assignment" .)