Top seat alongside head of trade (5)
I believe the answer is:
trump
'top' is the definition.
(thesaurus)
'seat alongside head of trade' is the wordplay.
'seat' becomes 'rump' ('rump' can be a synonym of 'seat'**).
'alongside' is a charade indicator (letters next to each other).
'head of' indicates taking the first letters.
The first letter of 'trade' is 't'.
'rump' after 't' is 'TRUMP'.
(Other definitions for trump that I've seen before include "Brick" , "Blast!" , "Reliable type" , "Ruff at cards" , "Surpass" .)