At liberty to mention no subscription is required (4,2,3)
I believe the answer is:
free to air
'mention no subscription is required' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'at liberty to' is the wordplay.
I cannot really understand how this works, but
'to' is found in the answer.
'at liberty' could be 'free' (similar in meaning) and 'free' is present in the answer.
The remaining letters 'air' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for free to air that I've seen before include "certain broadcasts" , "sort of programming" .)