Mathematician prompt to withdraw cover (6)
I believe the answer is:
euclid
'mathematician' is the definition.
(I know that Euclid is a geometer)
'prompt to withdraw cover' is the wordplay.
'prompt' becomes 'cue' (cue can mean a prompt or signal).
'to withdraw' says the letters should be written backwards.
'cover' becomes 'lid' (lid is a kind of cover).
'cue' backwards is 'euc'.
'euc'+'lid'='EUCLID'
(Other definitions for euclid that I've seen before include "Greek mathematician, 3rd century BC" , "Greek philosopher" , "Early Greek geometrician" , "Old Greek master of geometry" , "Greek mathematician, d. mid-3rd century BC" .)