Go, feeling one’s leaving for long time (4,8)
I believe the answer is:
life sentence
'for long time' is the definition.
(a long time in prison)
'go feeling one's' is the wordplay.
'go' becomes 'life' (go can mean life or energy).
'feeling one' becomes 'sentence' (I can't justify this - if you can you should believe this answer much more).
'life'+'sentence'='LIFE SENTENCE'
'leaving' is the link.
(Other definitions for life sentence that I've seen before include "short extract from American magazine" , "Long term of imprisonment" , "Long prison stay" , "Judges may confirm such" , "Long-term punishment" .)