Leo solemn about this type of flatfish (5,4)
I believe the answer is:
lemon sole
'this type of flatfish' is the definition.
'leo solemn about' is the wordplay.
'about' indicates an anagram.
'leo'+'solemn'='leosolemn'
'leosolemn' is an anagram of 'LEMON SOLE'.
(Other definitions for lemon sole that I've seen before include "Common flatfish" , "Tasty plaice of citrus origin?" , "Edible flatfish" , "Another tasty flatfish like lone citrus fruit" , "swimmer" .)