At no time lost nerve (5)
I believe the answer is:
never
'at no time' is the definition.
('never' can be a synonym of 'at no time')
'lost nerve' is the wordplay.
'lost' indicates an anagram.
'nerve' with letters rearranged gives 'NEVER'.
(Other definitions for never that I've seen before include "Under no circumstances" , "when hell freezes over?" , "Certainly not!" , "Not ever" , "Surely not" .)