Excuse a student with one book -- one! (5)
I believe the answer is:
alibi
'excuse' is the definition.
(alibiing is a kind of excusing)
'a student with one book one' is the wordplay.
'student' becomes 'l'.
'with' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'book' becomes 'b'.
'one' becomes 'i' (Roman numeral).
'a'+'l'+'i'+'b'+'i'='ALIBI'
(Other definitions for alibi that I've seen before include "Proof of not being at the scene of the crime" , "Proof of being elsewhere at the time of a crime." , "Legal defence proving innocence" , "Excuse for failure" , "in order to prove I was elsewhere" .)