Turns up a long spring is a favourite in the Spanish restaurant business (6)
I believe the answer is:
paella
'favourite in the spanish restaurant business' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'turns up a long spring' is the wordplay.
'turns up' says the letters should be written backwards.
'long' becomes 'l'.
'spring' becomes 'leap' ('leap' can be a synonym of 'spring'**).
'a'+'l'+'leap'='alleap'
'alleap' backwards is 'PAELLA'.
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for paella that I've seen before include "Spanish dish of rice, shellfish, etc" , "Span. rice dish" , "Dish (not English)" , "Al, leap into the Spanish dish" , "Saffron-flavoured rice dish" .)