Gosh, it's old-fashioned! (4)
I believe the answer is:
egad
'gosh' is the definition.
Both the definition and answer are interjections.
Perhaps you can see an association between them that I don't see?
'it's old-fashioned' is the wordplay.
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'old fashioned' becomes 'gad' (I can't justify this - if you can you should believe this answer much more).
'e'+'gad'='EGAD'
Can you help me to learn more?
(Other definitions for egad that I've seen before include "My word once" , "Old oath" , "Forgotten curse" , "Christ" , "Mild oath (archaic)" .)