Third in milk can collected by white tongue (7)
I believe the answer is:
walloon
'tongue' is the definition.
(dialect of French)
'third in milk can collected by white' is the wordplay.
'third in milk' becomes 'l' (3rd letter of 'milk').
'can' becomes 'loo' (slang).
'collected by' indicates putting letters inside.
'white' becomes 'wan' (very pale).
'l'+'loo'='lloo'
'lloo' inserted into 'wan' is 'WALLOON'.
(Other definitions for walloon that I've seen before include "Belgian dialect" , "Southern Belgian" , "European" , "Native of S Belgium or N France area" , "Francophone" .)