American escaping from even church's grasp (6)
I believe the answer is:
Here is my best explanation:
'grasp' is the definition.
('clench' can be a synonym of 'grasp')
'american escaping from even church's' is the subsidiary indication.
I cannot really see how this works, but
'church' could be 'ch' (common abbreviation) and 'ch' is present in the answer.
This explanation may well be incorrect...
(Other definitions for clench that I've seen before include "Hold in a tight grasp", "Settle; embrace", "Get a grip on", "Close teeth or fingers tightly", "Squeeze together tightly", "Grit", "Grip tightly", "Grasp firmly", "Close fingers into a tight ball" and "Hold tightly and firmly".)