Yours is the one going in to be repaired, then (5)
I believe the answer is:
'yours' is the definition.
(I've seen this in another clue)
'the one going in to be repaired then' is the wordplay.
'the one' becomes 'i' (Roman numeral).
'going in' means one lot of letters goes inside another.
'to be repaired' indicates anagramming the letters.
'then' anagrammed gives 'thne'.
'i' going into 'thne' is 'THINE'.
'is' is the link.
(Another definition for thine that I've seen is " Yours (archaic)".)