A librarian has one for an excuse (5)
I believe the answer is:
alibi
'for an excuse' is the definition.
(excuse given by a suspect in an investigation)
'a librarian has one' is the wordplay.
'librarian' becomes 'lib' (abbreviation for librarian).
'has' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'a'+'lib'+'i'='ALIBI'
(Other definitions for alibi that I've seen before include "it may help you to get off" , "Reason often given to exculpate someone" , "Legal defence proving innocence" , "Court evidence showing one was elsewhere" , "It shows one wasn't at crime scene" .)