What’s blue and quite sour when cooked (9)
I believe the answer is:
turquoise
'what's blue' is the definition.
(turquoise is a kind of blue)
'quite sour when cooked' is the wordplay.
'when cooked' indicates anagramming the letters.
'quite'+'sour'='quitesour'
'quitesour' with letters rearranged gives 'TURQUOISE'.
'and' is the link.
(Other definitions for turquoise that I've seen before include "Greenish-blue colour" , "Bluey-green" , "Sort of blue" , "Blue-green" , "Greenish-blue stone" .)