Let nine out, being merciful (7)
I believe the answer is:
lenient
'merciful' is the definition.
(synonyms)
'let nine out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'let'+'nine'='letnine'
'letnine' anagrammed gives 'LENIENT'.
'being' is the link.
(Other definitions for lenient that I've seen before include "Indulgent, lax" , "'Indulgent, easy (7)'" , "Inclined to let people off lightly" , "Showing tolerance" , "Tolerant, rather than strict" .)