It’s a matter of choice (6)
I believe the answer is:
ballot
'choice' is the definition.
(ballot is a kind of choice)
'it's a matter' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'matter' could be 'o' (O is an example) and 'o' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for ballot that I've seen before include "Secret vote" , "Vote or suffrage" , "Poll" , "Lottery (eg, for tickets)" , "The process of exercising 20 Down" .)