Happy prisoner’s first to get let out (7)
I believe the answer is:
pleased
'happy' is the definition.
(synonyms)
'prisoner's first to get let out' is the wordplay.
'prisoner's first' becomes 'p' (1st letter of 'prisoner').
'to get' means one lot of letters go next to another.
'let out' becomes 'leased' (as in leasing or letting out property).
'p'+'leased'='PLEASED'
(Other definitions for pleased that I've seen before include "Euphoric" , "Content" , "Happy, satisfied" , "Well satisfied" , "Feeling satisfaction" .)