Jack, for instance, occasionally on board (4)

I believe the answer is:

card

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'jack for instance' is the definition.
(jack is a type of playing card)

I don't understand the remainder of the clue.

(Other definitions for card that I've seen before include "showing heart, say" , "What may be used for greeting" , "One taken in hand" , "programme at race meeting" , "Christmas greeting" .)

I've seen this clue in the Evening Standard.

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