Nile net loosened: that's merciful (7)
I believe the answer is:
lenient
'merciful' is the definition.
(synonyms)
'nile net loosened' is the wordplay.
'loosened' is an anagram indicator.
'nile'+'net'='nilenet'
'nilenet' anagrammed gives 'LENIENT'.
'that's' is the link.
(Other definitions for lenient that I've seen before include "Mild and tolerant" , "Not severe, merciful" , "Inclined to let people off lightly" , "Tolerant, merciful" , "Clement" .)