Gosh, a bit of fun on a Greek island! (5)
I believe the answer is:
corfu
'a greek island' is the definition.
(Corfu is a Greek island)
'gosh a bit of fun' is the wordplay.
I cannot quite see how this works, but
'gosh' could be 'cor' (I've seen this in other clues) and 'cor' is found in the answer.
This explanation may well be incorrect...
'on' acts as a link.
Can you help me to learn more?
(Other definitions for corfu that I've seen before include "Holiday location" , "holiday venue!" , "Island off west of Greece, popular for holidays" , "Greece's 'Kerkyra'" , "Major holiday island off west coast of Greece" .)