It can be tied then, ie switched, to reinforce (10)

I believe the answer is:

strengthen

'reinforce' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for strengthen that I've seen before include "rally" , "Toughen up" , "Bolster" , "Build up" , "Brace" .)

I've seen this clue in The Guardian.

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