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.
Maybe you can see a link between them that I don't see?
'that of always wearing one hat' is the wordplay.
'that of always' becomes 'bi' (I can't explain this - if you can you should believe this answer much more).
'wearing' means one lot of letters goes inside another.
'bi' going inside '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" .)