Send model fife off to be made rigid (9)
I believe the answer is:
stiffened
'made rigid' is the definition.
The definition and answer can be both related to change as well as being past participle verbs.
Perhaps you can see an association between them that I can't see?
'send model fife off' is the wordplay.
'model' becomes 'T' (Ford Model T car).
'off' indicates an anagram.
'send'+'t'+'fife'='sendtfife'
'sendtfife' anagrammed gives 'STIFFENED'.
'to be' is the link.
Can you help me to learn more?
(Other definitions for stiffened that I've seen before include "Reinforced; hardened" , "Grew rigid" , "given support" .)