Make strong and durable (7)

I believe the answer is:

toughen

'make strong and durable' is the definition.
(I've seen this before)

This is the entire clue.

(Other definitions for toughen that I've seen before include "Harden, strengthen" , "Make strong and durable" , "Add resistance to" , "Increase stiffness" , "Make more robust" .)

I've seen this clue in The Irish Times.

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