Harry’s book, Spare (6)
I believe the answer is:
bother
'harry's' is the definition.
(thesaurus)
'book spare' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'spare' becomes 'other' (other can mean spare or alternative).
'b'+'other'='BOTHER'
(Other definitions for bother that I've seen before include "Hassle - drat!" , "Trouble, fuss" , "Badger" , "Fuss and annoyance" , "Fuss and irritant" .)