Old writer by entrance showing general availability (4,6)
I believe the answer is:
open access
'general availability' is the definition.
The definition suggests a singular noun which matches the answer.
'old writer by entrance' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'writer' becomes 'pen' (something which writes).
'by' means one lot of letters go next to another.
'entrance' becomes 'access' (I've seen this before).
'o'+'pen'+'access'='OPEN ACCESS'
'showing' acts as a link.
Can you help me to learn more?
(Another definition for open access that I've seen is " which library offers".)