Old soldier's singular dance (6)

I believe the answer is:

lancer

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'old soldier's' is the definition.
'lancer' can be an answer for 'soldier's' (lancer is a kind of soldier). I'm not certain of the 'old' bit.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for lancer that I've seen before include "Spearman (as in dance?)" , "Cavalry soldier" , "Former cavalryman" .)

I've seen this clue in the Evening Standard.

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