Group of MPs inclined, mostly, to carry vote for pledge (6,9)
I believe the answer is:
select committee
'group of mps' is the definition.
(UK political term)
'inclined mostly to carry vote for pledge' is the wordplay.
'inclined' becomes 'steep'.
'mostly' means to remove the last letter.
'to carry' means one lot of letters goes inside another.
'vote for' becomes 'elect' (I've seen this before).
'pledge' becomes 'commit'.
'steep' with its final letter taken away is 'stee'.
'elect'+'commit'='electcommit'
'stee' going around 'electcommit' is 'SELECT COMMITTEE'.
(Other definitions for select committee that I've seen before include "Group of MPs" , "parliamentary group" .)