That of always wearing one hat out of two? (5)
I believe the answer is:
habit
'out of two?' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'that of always wearing one hat' is the wordplay.
'that of always' becomes 'bi' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'wearing' means one lot of letters goes inside another.
'bi' put into 'hat' is 'HABIT'.
Can you help me to learn more?
(Other definitions for habit that I've seen before include "Monk's robe or repetitive action" , "Regular practice" , "It's worn in order" , "Ingrained behaviour" , "Custom; riding dress" .)