No – back in the lead, running on the outside (6)
I believe the answer is:
denial
'no' is the definition.
(I've seen this before)
'back in the lead running on the outside' is the wordplay.
'back' says the letters should be written backwards.
'running' indicates anagramming the letters.
'on the outside' indicates putting letters inside.
'in' reversed gives 'ni'.
'lead' is an anagram of 'deal'.
'ni' going inside 'deal' is 'DENIAL'.
(Other definitions for denial that I've seen before include "Repudiation" , "Refusal to acknowledge" , "Declaration that something is untrue" , "Refusal, abnegation" , "Renunciation" .)