Milk added at the start contaminating tea (7)
I believe the answer is:
extract
'milk' is the definition.
(thesaurus)
'added at the start contaminating tea' is the wordplay.
'added' becomes 'extra' ('extra' can be similar in meaning to 'added').
'at the start' suggests taking the first letters.
The first letters of 'contaminating tea' is 'ct'.
'extra'+'ct'='EXTRACT'
(Other definitions for extract that I've seen before include "Clipping" , "Short passage from a longer work" , "Passage selected from a larger work" , "Piece taken from a longer work" , "essence" .)