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' 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!" .)