Suggestion upset top mathematician (6)
I believe the answer is:
euclid
'mathematician' is the definition.
(Euclid is an example)
'suggestion upset top' is the wordplay.
'suggestion' becomes 'cue' (I've seen this in other clues).
'upset' shows that the letters should be reversed in order.
'top' becomes 'lid' (lid is a kind of top).
'cue' back-to-front is 'euc'.
'euc'+'lid'='EUCLID'
(Other definitions for euclid that I've seen before include "6 mathematician" , "Greek mathematician c. three hundred BC" , "Great Greek master of geometry" , "Old mathematician" , "Greek philosopher" .)