Not once was nerve lost (5)
I believe the answer is:
never
'not once' is the definition.
(I've seen this before)
'nerve lost' is the wordplay.
'lost' indicates an anagram.
'nerve' anagrammed gives 'NEVER'.
'was' acts as a link.
(Other definitions for never that I've seen before include "Do not give up!" , "No way" , "At no time in the past or future" , "On no account" , "Not in a million years!" .)