Blow wind instrument outside back of inn (6)
I believe the answer is:
bungle
'blow' is the definition.
(to blow something is to bungle or ruin it)
'wind instrument outside back of inn' is the wordplay.
'wind instrument' becomes 'bugle' (bugle is a kind of wind instrument).
'outside' means one lot of letters goes inside another.
'back of inn' becomes 'n' (I can't explain this - if you can you should believe this answer much more).
'bugle' enclosing 'n' is 'BUNGLE'.
(Other definitions for bungle that I've seen before include "Tackle clumsily and fail" , "Badly handle" , "Do something carelessly or badly" , "Clumsy mistake" , "Botch; failure" .)