Month in which prisoner acquires a second language (8)
I believe the answer is:
malagasy
'language' is the definition.
(I've seen this before)
'month in which prisoner acquires a second' is the wordplay.
'month' becomes 'may' (May is an example).
'in which' means one lot of letters goes inside another.
'prisoner' becomes 'lag' (I've seen this before).
'acquires a second' becomes 'as' (this might be a standard abbreviation which I don't know about).
'lag'+'as'='lagas'
'may' enclosing 'lagas' is 'MALAGASY'.
Can you help me to learn more?
(Other definitions for malagasy that I've seen before include "African islander" , "Asian" , "language" , "old African republic" .)