The best prison, we hear, will offer outdoor excursion (6)
I believe the answer is:
picnic
'outdoor excursion' is the definition.
'the best prison we hear' is the wordplay.
'the best' becomes 'pic' (I can't justify this - if you can you should give a lot more credence to this answer).
'prison' becomes 'nick' (slang term for prison).
'we hear' shows a homophone (sound like).
'nick' is a homophone of 'nic'.
'pic'+'nic'='PICNIC'
'will offer' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
(Other definitions for picnic that I've seen before include "food for consumption outside" , "outing" , "Informal meal taken in the open air" , "Meal eaten outside" , "Informal meal in open air for pleasure" .)