Canned - as sardines are (5)
I believe the answer is:
oiled
'canned as' is the definition.
The definition and answer can be both related to change as well as being past participle verbs.
Maybe you can see an association between them that I don't see?
'sardines are' is the wordplay.
'sardines' becomes 'oi' (I can't explain this - if you can you should give a lot more credence to this answer).
'are' becomes 'led' (leading is a kind of being).
'oi'+'led'='OILED'
Can you help me to learn more?
(Other definitions for oiled that I've seen before include "Lubricated or drunk" , "greasy" , "Besotted" , "Oldie (anag)" , "Cut" .)