Taste underlying mustard and mace ingredients first of all (5)
I believe the answer is:
umami
'taste' is the definition.
(I've seen this before)
'underlying mustard and mace ingredients first of all' is the wordplay.
'first of all' says to take the initial letters.
The initial letters of 'underlying mustard and mace ingredients' is 'UMAMI'.
(Other definitions for umami that I've seen before include "meaty taste" , "Newly discovered taste" , "Japanese word for pleasant savoury taste" , "Savoury flavour" , "Savoury taste sensation produced by glutamates - I, a mum (anag)" .)