Jolly tea in carriage seat (8)
I believe the answer is:
armchair
'seat' is the definition.
(I know that Armchair is a seat)
'jolly tea in carriage' is the wordplay.
'jolly' becomes 'RM' (jolly can mean a Royal Marine).
'tea' becomes 'cha' (cha is a type of tea).
'in' means one lot of letters goes inside another.
'carriage' becomes 'air' (both can mean someone's stance or posture).
'rm'+'cha'='rmcha'
'rmcha' going inside 'air' is 'ARMCHAIR'.
(Other definitions for armchair that I've seen before include "It's comfortable to sit in" , "Piece of furniture" , "Place for critic" , "Seat with side supports" , "Seat to relax in" .)