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 wordplay.
'knight' becomes 'n' (chess).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'breaking' means one lot of letters goes inside another.
'fast' becomes 'lent' (I've seen this before).
'nie' inserted into 'lent' is 'LENIENT'.
'of' is the link.
(Other definitions for lenient that I've seen before include "Soft-hearted", "Merciful or easygoing", "forbearing", "Indulgent, lax", "Tolerant, rather than strict", "Tolerant, merciful", "Not severe in punishing", "Compassionate", "Not severe, merciful" and "Easy, soft".)