Senior citizens are relatively so (5)

I believe the answer is:

older

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'senior' is the definition.
(senior can mean the older of two people)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for older that I've seen before include "More senior" , "Of greater age (O, not E)" , "Getting on" , "More elderly" , "as we all will be tomorrow" .)

I've seen this clue in the Evening Standard.

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