Book on island, good, important (3)
I believe the answer is:
big
'important' is the definition.
('big' can be a synonym of 'important')
'book on island good' is the wordplay.
'book' becomes 'b'.
'on' is a charade indicator (letters next to each other).
'island' becomes 'i'.
'good' becomes 'g' (abbreviation).
'b'+'i'+'g'='BIG'
(Other definitions for big that I've seen before include "Massive" , "Important" , "Of considerable size" , "Large; generous" , "Circus tent" .)