Delighted first of prisoners let out (7)
I believe the answer is:
pleased
'delighted' is the definition.
(similar in meaning)
'first of prisoners let out' is the wordplay.
'first of prisoners' becomes 'p' (1st letter of 'prisoners').
'let out' becomes 'leased' (I've seen this in another clue).
'p'+'leased'='PLEASED'
(Other definitions for pleased that I've seen before include "Well satisfied" , "Glad" , "Grateful, delighted" , "Content" , "Gratified" .)