Not once was nerve lost (5)
I believe the answer is:
never
'not once' is the definition.
(I've seen this in another clue)
'nerve lost' is the wordplay.
'lost' is an anagram indicator.
'nerve' is an anagram of 'NEVER'.
'was' is the 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!" .)