Far from being a century, it's simply beyond practical availability (3,2)
I believe the answer is:
not on
'simply beyond practical availability' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'far from being a century' is the wordplay.
I cannot quite see how this works, but
'from' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
'century' could be 'ton' (both can mean one hundred of something) and 'ton' is present in the answer.
A single letter 'n' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'it's' is the link.
Can you help me to learn more?
(Other definitions for not on that I've seen before include "Colloquially, cannot be tolerated or accepted" , "It's unfeasible" , "Improper - impossible" , "Informally, can't be accepted or tolerated" , "Unacceptable (colloq.)" .)