Mathematician's prompt returning cap (6)
I believe the answer is:
euclid
'mathematician's' is the definition.
(Euclid is an example)
'prompt returning cap' is the wordplay.
'prompt' becomes 'cue' (cue can mean a prompt or signal).
'returning' shows that the letters should be reversed in order.
'cap' becomes 'lid' (eg the lid of a bottle).
'cue' written backwards gives 'euc'.
'euc'+'lid'='EUCLID'
(Other definitions for euclid that I've seen before include "Early Greek geometrician" , "Greek mathematician c. three hundred BC" , "Alexandrian mathematician" , "Greek geometer" , "Old Greek master of geometry" .)