Signal over top mathematician (6)
I believe the answer is:
euclid
'mathematician' is the definition.
(Euclid is an example)
'signal over top' is the wordplay.
'signal' becomes 'cue' (as in an actor's cue).
'over' is a reversal indicator.
'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 "Great Greek master of geometry" , "Alexandrian mathematician" , "Great Greek mathematician, notable for geometry" , "Ancient Greek mathematician" , "Ancient Greek master of geometry" .)