Just book early for a change (6)
'just' is the definition.
'book early for a change' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'for a change' is an anagram indicator.
'early' anagrammed gives 'arely'.
(Other definitions for barely that I've seen before include "how streakers like to act?", "Not quite", "Just about or scantily", "how naturists exist", "how babies are born?".)