Tolerant of knight that is breaking fast (7)
I believe the answer is:
Here is my best explanation:
'tolerant' is the definition.
('lenient' can be a synonym of 'tolerant')
'knight that is breaking fast' is the subsidiary indication.
'knight' becomes 'n' (chess).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'breaking' indicates putting letters inside.
'fast' becomes 'lent'.
'nie' placed into 'lent' is 'lenient'.
'of' is the link.
(Other definitions for lenient that I've seen before include "Indulgent, lax", "Tolerant, merciful", "Not severe, merciful", "Not severe in punishing", "Tolerant, rather than strict", "Soft on the rules", "Clement", "Merciful, tolerant", "Mild and tolerant" and "Merciful or easygoing".)