Let off the hook - the United Nations hit a snag (7)
I believe the answer is:
unhitch
'let off the hook the united nations' is the definition.
I can't judge whether this defines the answer.
'hit a snag' is the wordplay.
'hit' becomes 'un' (I can't justify this - if you can you should believe this answer much more).
'a snag' becomes 'hitch' (synonyms).
'un'+'hitch'='UNHITCH'
Can you help me to learn more?
(Other definitions for unhitch that I've seen before include "free?" , "Release" , "Remove from a harness" , "divorce?" , "Part" .)