Inject copper into deformed toenail (9)
I believe the answer is:
inoculate
'inject' is the definition.
(inoculating is a kind of injecting)
'copper into deformed toenail' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'into' means one lot of letters goes inside another.
'deformed' is an anagram indicator.
'toenail' anagrammed gives 'inolate'.
'cu' put into 'inolate' is 'INOCULATE'.
(Other definitions for inoculate that I've seen before include "Induce immunity" , "Treat with a vaccine or serum" , "give someone the needle" , "Introduce an idea or attitude into the mind of: immunise" , "Immunise, vaccinate" .)