Not huge but shaped to become robust (7)

I believe the answer is:

toughen

I'm a little stuck... Click here to teach me more about this clue!

'shaped to become robust' is the definition.
I can't tell whether this definition defines the answer.

'not huge' is the wordplay.
'not huge' can be anagrammed to 'TOUGHEN'.
But, I am not clear how this is indicated.

'but' acts as a link.

Can you help me to learn more?

(Other definitions for toughen that I've seen before include "Fortify" , "Make more resilient" , "Strengthen, season" , "Make more robust" , "Make strong and durable" .)

I've seen this clue in The Globe and Mail.

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