Note: always book returns (5)
I believe the answer is:
breve
'note' is the definition.
(musical note)
'always book returns' is the wordplay.
'always' becomes 'ever' (synonyms).
'book' becomes 'b' (abbreviation for book).
'returns' shows that the letters should be reversed in order.
'ever'+'b'='everb'
'everb' written backwards gives 'BREVE'.
(Other definitions for breve that I've seen before include "Accent placed over a vowel" , "Accent - musical note" , "Note with the time value of eight crotchets" , "(In music) long note" , "Long musical note" .)