Joins in a bit of fun, knotting ties (6)
I believe the answer is:
unites
'joins' is the definition.
(uniting is a kind of joining)
'bit of fun knotting ties' is the wordplay.
'bit of fun' becomes 'un' (I can't explain this - if you can you should believe this answer much more).
'knotting' is an anagram indicator (I've seen 'knot' mean this (letters are tangled up)).
'ties' anagrammed gives 'ites'.
'un'+'ites'='UNITES'
'in a' acts as a link.
Can you help me to learn more?
(Other definitions for unites that I've seen before include "Becomes one" , "Comes together as one" , "Comes together for a common purpose" , "(old) money?" , "Merges" .)