Group of MPs inclined, mostly, to carry vote for pledge (6,9)
I believe the answer is:
Here is my best explanation:
'group of mps' is the definition.
(UK political term)
'inclined mostly to carry vote for pledge' is the subsidiary indication.
'inclined mostly' becomes 'stee' (I can't explain this - if you can you should give a lot more credence to this answer).
'to carry' indicates putting letters inside.
'vote for' becomes 'elect' (I've seen this before).
'pledge' becomes 'commit'.
'stee' enclosing 'electcommit' is 'selectcommittee'.
(Other definitions for select committee that I've seen before include "Group of MPs" and "parliamentary group".)