Beatles movie mostly set on island somewhere in Finland (8)
I believe the answer is:
helsinki
'in finland' is the definition.
('Helsinki' is part of 'finland')
'beatles movie mostly set on island somewhere' is the wordplay.
'beatles movie' becomes 'help' (I've seen this in another clue).
'mostly' means to remove the last letter (most but not all of the word).
'set' becomes 'sink' (to go lower).
'on' means one lot of letters go next to another.
'island somewhere' becomes 'i' (I can't justify this - if you can you should give a lot more credence to this answer).
'help' with its last letter taken off is 'hel'.
'hel'+'sink'+'i'='HELSINKI'
Can you help me to learn more?
(Other definitions for helsinki that I've seen before include "Is he link to a European city?" , "Capital of Finland" , "Finland's capital" , "Finland's largest city" , "North European capital" .)