One of those on and off romances? (6)

I believe the answer is:

serial

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'one' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?

I can't explain the remainder of the clue.

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(Other definitions for serial that I've seen before include "Continuous, perhaps like mulitple killer ( 6)" , "TV drama in instalments" , "Continuous story set in Israel" , "Story made up of consecutive parts" , "Story published in instalments" .)

I've seen this clue in the Evening Standard.

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