Suggestion backed top mathematician (6)
I believe the answer is:
euclid
'mathematician' is the definition.
(Euclid is an example)
'suggestion backed top' is the wordplay.
'suggestion' becomes 'cue' (I've seen this in other clues).
'backed' says the letters should be written backwards.
'top' becomes 'lid' (lid is a kind of top).
'cue' written backwards gives 'euc'.
'euc'+'lid'='EUCLID'
(Other definitions for euclid that I've seen before include "6 mathematician" , "Ancient Greek mathematician" , "Greek philosopher" , "Ancient Greek master of geometry" , "Old Greek master of geometry" .)