Prolific writer Asimov (5)
I believe the answer is:
isaac
'prolific writer asimov' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for isaac that I've seen before include "Singer" , "Jewish name?" , "designated victim" , "- Newton, Eng. physicist and mathematician" , "OT near-sacrificial victim" .)