Back including 'monsieur' in Asian language (5)
I believe the answer is:
tamil
'asian language' is the definition.
(I've seen this before)
'back including monsieur' is the wordplay.
'back' becomes 'tail' (tail is a kind of back).
'including' means one lot of letters goes inside another.
'monsieur' becomes 'm'.
'tail' going around 'm' is 'TAMIL'.
'in' is the link.
(Other definitions for tamil that I've seen before include "An official language of Sri Lanka" , "Sri Lankan" , "Language of southern India and Sri Lanka" , "Asian language" , "S. India/Sri Lankan person, tongue" .)