Top copy with ten underneath (4)
I believe the answer is:
apex
'top' is the definition.
(thesaurus)
'copy with ten underneath' is the wordplay.
'copy with' becomes 'ape' ('ape' can be a synonym of 'copy'. I am not sure about the 'with' bit.**).
'ten' becomes 'x' ('X' can be a synonym of 'ten').
'underneath' says to put letters next to each other.
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "climax" , "Culmination" , "Peak, vertex" , "crown" , "Apogee, summit" .)