Cheese is extravagant in one of the Costas (7)
I believe the answer is:
ricotta
'cheese' is the definition.
(ricotta is a type of cheese)
'extravagant in one of the costas' is the wordplay.
'extravagant' becomes 'OTT' (abbreviation for 'over the top').
'in' means one lot of letters goes inside another.
'one of the costas' becomes 'rica' (I can't explain this - if you can you should believe this answer much more).
'ott' put inside 'rica' is 'RICOTTA'.
'is' is the link.
Can you help me to learn more?
(Other definitions for ricotta that I've seen before include "may come from Italian press?" , "one brought on board?" , "One soft cheese" , "dairy produce" , "Ital. cheese" .)
