Join one in a new tune (5)
I believe the answer is:
unite
'join' is the definition.
(I know that unite is a kind of join)
'one in a new tune' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'a new' indicates an anagram.
'tune' with letters rearranged gives 'unte'.
'i' going into 'unte' is 'UNITE'.
(Other definitions for unite that I've seen before include "Gather together as one" , "old coin" , "pool resources" , "Join forces" , "Put together" .)