Excuse from party installed in first-class (5)
I believe the answer is:
alibi
'excuse' is the definition.
(excuse given by a suspect in an investigation)
'party installed in first-class' is the wordplay.
'party' becomes 'lib' (abbreviation for Liberal, uk political party).
'installed in' means one lot of letters goes inside another.
'first class' becomes 'ai' (A1 - top quality).
'lib' inserted within 'ai' is 'ALIBI'.
'from' acts as a link.
(Other definitions for alibi that I've seen before include "Lawyer's weapon, Latin for 'elsewhere'" , "excuse given" , "Defence used by an accused person" , "Evidence one was not present" , "Legal term from the Latin 'elsewhere'" .)