Untie tricky knot (5)
I believe the answer is:
unite
'knot' is the definition.
(thesaurus)
'untie tricky' is the wordplay.
'tricky' indicates an anagram.
'untie' is an anagram of 'UNITE'.
(Other definitions for unite that I've seen before include "Group of workers" , "Gather together as one" , "Become a single whole" , "Bring together, become one" , "Combine - join" .)