Tune that comes back in variation (3)
I believe the answer is:
air
'tune' is the definition.
('air' can be a synonym of 'tune')
'comes back in variation' is the wordplay.
'comes back' says the letters should be written in reverse.
'in' says the answer is hidden in the clue.
'ria' is hidden in the letters of 'variation'.
'ria' back-to-front is 'AIR'.
'that' is the link.
(Other definitions for air that I've seen before include "Broadcast - song" , "Light breeze" , "in which birds fly around" , "Dry; tune" , "Ventilate, give voice to" .)