Out of prison, almost all are very good (5)
I believe the answer is:
excel
'all are very good' is the definition.
I can't tell whether this definition defines the answer.
'out of prison almost' is the wordplay.
'out of' becomes 'ex'.
'prison' becomes 'cell' (I've seen this before).
'almost' means to remove the last letter (most of the word but not all of it).
'cell' with its last letter taken off is 'cel'.
'ex'+'cel'='EXCEL'
Can you help me to learn more?
(Other definitions for excel that I've seen before include "Do better than, give great performance" , "Outrival" , "well above average" , "Be the best" , "Top" .)