Upholsterers add leather to cover seat (6)
I believe the answer is:
saddle
'seat' is the definition.
(I know that saddle is a type of seat)
'upholsterers add leather to cover' is the wordplay.
'to cover' indicates the answer is hidden within the clue (I've seen 'covering' mean this).
'SADDLE' can be found hidden inside 'upholsterers add leather'.
(Other definitions for saddle that I've seen before include "Lumber (with); one on horse" , "Equestrian seat" , "Jockey's seat" , "Chafed by riding" , "Burden; seat" .)