Come Together: rocky tune grabs one (5)
I believe the answer is:
unite
'come together' is the definition.
(to unite is to come or join together)
'rocky tune grabs one' is the wordplay.
'rocky' indicates anagramming the letters.
'grabs' means one lot of letters goes inside another.
'one' becomes 'I' (Roman numeral).
'tune' with letters rearranged gives 'unte'.
'unte' going around 'i' is 'UNITE'.
(Other definitions for unite that I've seen before include "Join together, merge" , "Couple" , "Make one" , "Join forces" , "Gather together as one" .)