Old Vlad in the Netherlands climbing? Only it's flat! (5,4)
I believe the answer is:
lemon sole
'it's flat' is the definition.
(sole is a flatfish)
I don't understand how the remainder of the clue works.
(Other definitions for lemon sole that I've seen before include "Edible flatfish" , "swimmer" , "Another tasty flatfish like lone citrus fruit" , "Lose melon, find kind of plaice" , "Tasty plaice of citrus origin?" .)