Group of MPs choose to pledge support (6,9)
I believe the answer is:
select committee
'group of mps' is the definition.
(UK political term)
'choose to pledge support' is the wordplay.
'choose' becomes 'select' ('select' can be a synonym of 'choose'**).
'to pledge' becomes 'commit'.
'support' becomes 'tee' (I've seen this before).
'select'+'commit'+'tee'='SELECT COMMITTEE'
(Other definitions for select committee that I've seen before include "Group of MPs" , "parliamentary group" .)