Signal sent back by top Greek mathematician (6)
I believe the answer is:
euclid
'greek mathematician' is the definition.
(I've seen this before)
'signal sent back by top' is the wordplay.
'signal' becomes 'cue' (as in an actor's cue).
'sent back' shows that the letters should be reversed in order.
'by' is a charade indicator (letters next to each other).
'top' becomes 'lid' (lid is a kind of top).
'cue' in reverse letter order is 'euc'.
'euc'+'lid'='EUCLID'
(Other definitions for euclid that I've seen before include "mathematician appears" , "Old Greek master of geometry" , "Greek geometer" , "Greek father of geometry" , "Alexandrian mathematician" .)