Sign backing top mathematician (6)
I believe the answer is:
euclid
'mathematician' is the definition.
(Euclid is an example)
'sign backing top' is the wordplay.
'sign' becomes 'cue' (a cue is a sign or signal).
'backing' shows that the letters should be reversed in order.
'top' becomes 'lid' (lid is a kind of top).
'cue' reversed gives 'euc'.
'euc'+'lid'='EUCLID'
(Other definitions for euclid that I've seen before include "Greek father of geometry" , "Ancient Greek mathematician" , "Greek geometer" , "Greek mathematician, d. mid-3rd century BC" , "Book of geometry" .)