Guest wearing very informal shirt on island (7)
I believe the answer is:
invitee
'guest' is the definition.
(I know that guest can be written as invitee)
'wearing very informal shirt on island' is the wordplay.
'wearing very' becomes 'inv' (I can't justify this - if you can you should give a lot more credence to this answer).
'informal shirt' becomes 'tee' (I've seen this in another clue).
'on' says to put letters next to each other.
'island' becomes 'i' (geographical abbreviation).
'tee' after 'i' is 'itee'.
'inv'+'itee'='INVITEE'
Can you help me to learn more?
(Other definitions for invitee that I've seen before include "One asked to attend" , "special guest" , "visitor?" .)