Crossword Genius

Romantically involved, ditching Tom as secondary? (9)

Ross

I believe the answer is:

ancillary

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'secondary?' is the definition.
(thesaurus)

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(Other definitions for ancillary that I've seen before include "Additional" , "Providing necessary support" , "Supporting, subsidiary" , "Accessory, auxiliary" .)

I've seen this clue in The Independent.
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