Unusual pressure in seat (4)
I believe the answer is:
rump
'seat' is the definition.
(I know that seat can be written as rump)
'unusual pressure' is the wordplay.
'unusual' becomes 'rum' (I've seen this before).
'pressure' becomes 'p' (abbreviation).
'rum'+'p'='RUMP'
'in' is the link.
(Other definitions for rump that I've seen before include "Meat (from behind?)" , "Rear; buttocks" , "Inferior remains; cut of steak" , "Posterior" , "Fleshy hindquarters" .)