Support line missed by player in game (4)

I believe the answer is:

back

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'support' is the definition.
(to back is to support or endorse)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for back that I've seen before include "Rear -- endorse" , "Support a play - player" , "Support; in return" , "See 17" , "Series of popular time-travel movies" .)

I've seen this clue in The Independent.

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