Once dim, turning evil (7)
I believe the answer is:
demonic
'evil' is the definition.
(similar in meaning)
'once dim turning' is the wordplay.
'turning' indicates an anagram.
'once'+'dim'='oncedim'
'oncedim' with letters rearranged gives 'DEMONIC'.
(Other definitions for demonic that I've seen before include "Like evil spirits" , "diabolical" , "As if possessed by devil" , "Dime con (anag.)" , "Fiendish or satanic" .)