Back number possibly apt to be freely available (2,3)
I believe the answer is:
on tap
'be freely available' is the definition.
(I've seen this before)
'back number possibly apt' is the wordplay.
'back' is a reversal indicator.
'number' becomes 'no' (abbreviation for 'number').
'possibly' is an anagram indicator.
'no' reversed gives 'on'.
'apt' is an anagram of 'tap'.
'on'+'tap'='ON TAP'
'to' acts as a link.
(Other definitions for on tap that I've seen before include "Ready for immediate use" , "where C or H is seen?" , "handy" , "readily accessible" , "Constantly available like beer from keg" .)