River language this writer’s covering (6)
I believe the answer is:
mersey
'river' is the definition.
(English river)
'language this writer's covering' is the wordplay.
'language' becomes 'erse' (Erse is an example).
'this writer's' becomes 'my' (belonging to the person who set the crossword).
'covering' is an insertion indicator.
'erse' placed within 'my' is 'MERSEY'.
(Other definitions for mersey that I've seen before include "Liverpool's river" , "Route between Liverpool and the Wirral" , "River rising in Derbyshire and flowing into the Irish Sea" , "Scouse river" , "Runcorn's river" .)