Flavouring ingredient, mixture of melon and peach (5,5)
I believe the answer is:
lemon grass
'flavouring ingredient' is the definition.
'mixture of melon and peach' is the wordplay.
'mixture of' is an anagram indicator (I've seen this in other clues).
'and' says to put letters next to each other.
'peach' becomes 'grass' (both can mean to act as an informant).
'melon' anagrammed gives 'lemon'.
'lemon'+'grass'='LEMON-GRASS'
(Other definitions for lemon grass that I've seen before include "Source of oil" , "plant" , "Oriental aromatic used in cooking" .)