Dad's way is old-fashioned (5)
I believe the answer is:
passe
'old-fashioned' is the definition.
(synonyms)
'dad's way' is the wordplay.
'dad' becomes 'pa' (both can mean one's father).
'way' becomes 'sse' (I can't explain this - if you can you should give a lot more credence to this answer).
'pa'+'sse'='PASSE'
'is' is the link.
Can you help me to learn more?
(Other definitions for passe that I've seen before include "Old hat" , "No longer fashionable" , "Outdated" , "-- -partout" , "Old fashioned - in France" .)