Fastens bottom on top of seat (7)
I believe the answer is:
buttons
'fastens' is the definition.
(buttoning is a kind of fastening)
'bottom on top of seat' is the wordplay.
'bottom' becomes 'butt' ('butt' can be a synonym of 'bottom'**).
'top of' suggests taking the first letters.
The initial letter of 'seat' is 's'.
'butt'+'on'+'s'='BUTTONS'
(Other definitions for buttons that I've seen before include "a role in pantomime" , "Pantomime pageboy; fasteners" , "Baron Hardup's servant in Cinderella" , "Secures" , "Fastens" .)