Curious leader in parliamentary seat (4)
I believe the answer is:
rump
'seat' is the definition.
('rump' can be a synonym of 'seat')
'curious leader in parliamentary' is the wordplay.
'curious' becomes 'rum' ('rum' can be a synonym of 'curious').
'leader in' says to take the initial letters.
The first letter of 'parliamentary' is 'p'.
'rum'+'p'='RUMP'
(Other definitions for rump that I've seen before include "Inferior remains; cut of steak" , "historical Parliament" , "Unimportant remnant" , "The buttocks or hind-quarters" , "Bottom - cut of steak" .)