Work out tennis score divided by five hundred (6)
I believe the answer is:
deduce
'work out' is the definition.
(thesaurus)
'tennis score divided by five hundred' is the wordplay.
'tennis score' becomes 'deuce' (I've seen this before).
'divided by' indicates putting letters inside.
'five hundred' becomes 'd' ('D' can be a synonym of 'five hundred').
'deuce' enclosing 'd' is 'DEDUCE'.
(Other definitions for deduce that I've seen before include "Work out" , "Derive by reasoning" , "Arrive at reasoned conclusion from facts" , "Reason through logic" , "Arrive at conclusion through logic" .)