Put on Medusa's snakes (7)
I believe the answer is:
assumed
'put on' is the definition.
(synonyms)
'medusa's snakes' is the wordplay.
'snakes' indicates anagramming the letters (letters snake around).
'medusas' is an anagram of 'ASSUMED'.
(Other definitions for assumed that I've seen before include "Taken for granted - feigned" , "Surmised" , "Bogus" , "Accepted as true without actual proof" , "Took upon oneself" .)