Romantically involved, ditching Tom as secondary? (9)
I believe the answer is:
ancillary
'secondary?' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ancillary that I've seen before include "Additional" , "Providing necessary support" , "Supporting, subsidiary" , "Accessory, auxiliary" .)