Excuse deviation from script, but not second one (5)
I believe the answer is:
alibi
'excuse' is the definition.
(excuse given by a suspect in an investigation)
'deviation from script but not second one' is the wordplay.
'deviation from script' becomes 'ad lib' (as in an ad lib from an actor).
'but not' is a deletion indicator.
'second' becomes 'd'.
'one' becomes 'i' (Roman numeral).
'adlib' with 'd' taken away is 'alib'.
'alib'+'i'='ALIBI'
(Other definitions for alibi that I've seen before include "Pretext; court excuse" , "Claim put to court" , "A claim one was elsewhere when crime took place" , "Evidence of (or excuse for) being somewhere else at the time" , "Lawyer's weapon, Latin for 'elsewhere'" .)