Immature lout taken aback by silence on island (6)
I believe the answer is:
boyish
'immature' is the definition.
(similar in meaning)
'lout taken aback by silence on island' is the wordplay.
'lout' becomes 'yob' (both can mean a rude or criminal young person).
'taken aback' says the letters should be written in reverse.
'by' says to put letters next to each other.
'silence on island' becomes 'ish' (I can't justify this - if you can you should believe this answer much more).
'yob' in reverse letter order is 'boy'.
'boy'+'ish'='BOYISH'
Can you help me to learn more?
(Other definitions for boyish that I've seen before include "how immature youth is" , "like a young man" , "Youthful" , "Like a young lad" , "Juvenile" .)