A party has one excuse (5)
I believe the answer is:
'excuse' is the definition.
(excuse given by a suspect in an investigation)
'a party has one' is the wordplay.
'party' becomes 'lib' (abbreviation for Liberal, uk political party).
'has' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
(Other definitions for alibi that I've seen before include "Defence in court" , "Excuse, defence" , "Justification" , "Sort of explanation" , "can offer a valid defence against most charges" .)