Hardline United Nations committee with potential (14)
I believe the answer is:
uncompromising
'hardline' is the definition.
'united nations committee with potential' is the wordplay.
'united nations' becomes 'un' (synonyms).
'committee' becomes 'com' (abbreviation for committee).
'with potential' becomes 'promising'.
'un'+'com'+'promising'='UNCOMPROMISING'
(Other definitions for uncompromising that I've seen before include "firm" , "Obstinate" , "Not willing to make concessions" .)