Happy prisoners, only the first let out (7)
I believe the answer is:
pleased
'happy' is the definition.
(synonyms)
'prisoners only the first let out' is the wordplay.
'only the first' suggests taking the first letters.
'let out' becomes 'leased' (I've seen this before).
The initial letter of 'prisoners' is 'p'.
'p'+'leased'='PLEASED'
(Other definitions for pleased that I've seen before include "Glad" , "Feeling satisfaction" , "Grateful, delighted" , "like Punch?" , "Content" .)