Possibly spy on fashionable medium (7)
I believe the answer is:
psychic
'medium' is the definition.
(I've seen this in another clue)
'possibly spy on fashionable' is the wordplay.
'possibly' indicates an anagram.
'on' is a charade indicator (letters next to each other).
'fashionable' becomes 'chic' (chic means stylish or fashionable).
'spy' anagrammed gives 'psy'.
'psy'+'chic'='PSYCHIC'
(Other definitions for psychic that I've seen before include "Having extrasensory perception or occult powers" , "Of the soul or mind, or kind of fortune-teller" , "Sensitive to paranormal stimuli" , "One is apparently sensitive to the paranormal" , "Person with occult powers" .)