A roller primarily unused is so long (5)

I believe the answer is:

adieu

I'm a little stuck... Click here to teach me more about this clue!

'is so long' is the definition.
(I've seen this before)

'a roller primarily unused' is the wordplay.
'roller' becomes 'die' (I can't explain this - if you can you should give a lot more credence to this answer).
'primarily' suggests taking the first letters.
The first letter of 'unused' is 'u'.
'a'+'die'+'u'='ADIEU'

Can you help me to learn more?

(Other definitions for adieu that I've seen before include "Some say it when leaving" , "Auf wiedersehen" , "Vale" , "Parting word" , "I'm going" .)

I've seen this clue in The Hindu.

Want a hint initially instead of a full solution? Install my app