Sauce concealed in colossal sandwich (5)
I believe the answer is:
salsa
'sauce' is the definition.
(I've seen this before)
'concealed in colossal sandwich' is the wordplay.
'concealed in' indicates the answer is hidden within the clue.
'SALSA' is hidden in the letters of 'colossal sandwich'.
(Other definitions for salsa that I've seen before include "Latin-American dance" , "dancing?" , "dressing" , "Cuban dance music or spicy sauce" , "Sauce; Afro-Cuban dance" .)