What a stew - - that hidden flavour! (5)
I believe the answer is:
taste
'flavour' is the definition.
(I've seen this before)
'what a stew that hidden' is the wordplay.
'that hidden' indicates a hidden word.
'TASTE' is hidden within 'what a stew'.
(Other definitions for taste that I've seen before include "Appreciation, discernment" , "bit to eat" , "Partake of" , "Fine perception" , "Sample by eating or drinking a little" .)