Top seat taken by head of table (5)
I believe the answer is:
trump
'top' is the definition.
(thesaurus)
'seat taken by head of table' is the wordplay.
'seat' becomes 'rump' ('rump' can be a synonym of 'seat'**).
'taken by' means one lot of letters go next to another.
'head of' says to take the initial letters.
The first letter of 'table' is 't'.
'rump' after 't' is 'TRUMP'.
(Other definitions for trump that I've seen before include "Reliable type" , "reliable person" , "Trusty fellow (colloq.); ruff" , "The last one to announce" , "Card beating other suits" .)