Second top, say (5)
I believe the answer is:
speak
'say' is the definition.
(thesaurus)
'second top' is the wordplay.
'second' becomes 's' (common abbreviation).
'top' becomes 'peak' (I've seen this before).
's'+'peak'='SPEAK'
(Other definitions for speak that I've seen before include "give a lecture" , "express" , "Converse" , "Orate" , "Say something" .)