Available for selection as a matter of course (2,3,4)

'available for selection' is the definition.
(available to be selected from a menu)

'a matter of course' is the wordplay.
I cannot really see how this works, but
'matter' could be 'menu' (menu is a kind of matter) and 'menu' is found in the answer.
'of' could be 'o' and 'o' is found in the answer.
'course' could be 'n' and 'n' is present in the answer.
The remaining letters 'the' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

'as' is the link.