Child about to leave Spanish island (5)
I believe the answer is:
minor
'child' is the definition.
(synonyms)
'about to leave spanish island' is the wordplay.
'about' becomes 'ca' (circa).
'to leave' is a deletion indicator.
'spanish island' becomes 'Minorca' (Minorca is a Spanish island).
'minorca' with 'ca' removed is 'MINOR'.
(Other definitions for minor that I've seen before include "Not major" , "Little-known" , "Of small importance" , "Smaller - child" , "Person not yet of legal age" .)