Sprinkle the fourth ingredient into the stew (5)
I believe the answer is:
strew
'sprinkle' is the definition.
(I've seen this in another clue)
'fourth ingredient into the stew' is the wordplay.
'fourth ingredient' becomes 'r' (4th letter of 'ingredient').
'into' is an insertion indicator.
'r' inserted within 'stew' is 'STREW'.
'the' acts as a link.
(Other definitions for strew that I've seen before include "Scatter untidily" , "Scatter, spread around" , "Sprinkle" , "Scatter loosely" .)