Language used by loud Russians in retreat (4)
I believe the answer is:
urdu
'language' is the definition.
(Urdu is an example)
'loud russians in retreat' is the wordplay.
'in' indicates a hidden word.
'retreat' shows that the letters should be reversed in order.
'udru' is hidden within 'loud russians'.
'udru' backwards is 'URDU'.
'used by' is the link.
(Other definitions for urdu that I've seen before include "Form of Hindustani" , "Sanskrit language" , "speech" , "Indic language written in Persian script" , "Language spoken by Moslems in Pakistan and India" .)