Dull material with a happy ending (6)
I believe the answer is:
'dull' is the definition.
'material with a happy ending' is the wordplay.
I cannot quite understand how this works, but
'material' could be 'stuff' (synonyms) and 'stuff' is found in the answer.
A single letter 'y' remains which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for stuffy that I've seen before include "Lacking fresh air or excessively conventional" , "Close, airless" , "Close - conventional and dull" , "Priggish" , "Old-fashioned" .)