How cavalry may volunteer their free time? (4,2,6)

I believe the answer is:

make no charge

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'how cavalry may volunteer' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'free time?' is the wordplay.
'free' becomes 'make free' (synonyms).
'time?' becomes 'nocharge' (I am not sure about this - if you are sure you should believe this answer much more).
'make'+'nocharge'='MAKE NO CHARGE'

'their' acts as a link.

Can you help me to learn more?

I've seen this clue in The Globe and Mail.

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