Tolerant of one German keeling over during fast (7)
I believe the answer is:
lenient
'tolerant' is the definition.
('lenient' can be a synonym of 'tolerant')
'one german keeling over during fast' is the wordplay.
'one german' becomes 'ein' ('one' in German).
'keeling over' says the letters should be written backwards.
'during' means one lot of letters goes inside another.
'fast' becomes 'lent' (I've seen this before).
'ein' back-to-front is 'nie'.
'nie' placed into 'lent' is 'LENIENT'.
'of' is the link.
(Other definitions for lenient that I've seen before include "Soft-hearted" , "Mild and tolerant" , "Compassionate" , "Light" , "Not severe in punishing" .)