Sad in an old-fashioned way (6)
I believe the answer is:
triste
'sad' is the definition.
(I've seen this in another clue)
'in an old-fashioned way' is the wordplay.
'in' is an insertion indicator.
'an old fashioned' becomes 'trie' (I can't justify this - if you can you should give a lot more credence to this answer).
'way' becomes 'st' (abbreviation for street).
'trie' enclosing 'st' is 'TRISTE'.
Can you help me to learn more?
(Other definitions for triste that I've seen before include "Sad (arch)" , "Valse -- (Sibelius)" , "Old word for sad" , "Sad in France" .)