Past is glorious first off (5)
I believe the answer is:
olden
'past' is the definition.
(similar in meaning)
'glorious first off' is the wordplay.
'glorious' becomes 'golden'.
'first off' means to remove the first letter.
'golden' with its initial letter removed is 'OLDEN'.
'is' acts as a link.
(Other definitions for olden that I've seen before include "Bygone" , "Led on (anag)" , "as in time gone by" , "(Of days) in the long distant past!" , "Former, past" .)