Instrument accompanying wordless tune is boring (7)
I believe the answer is:
humdrum
'is boring' is the definition.
(I've seen this in another clue)
'instrument accompanying wordless tune' is the wordplay.
'instrument' becomes 'drum' (drum is a kind of instrument).
'accompanying' is a charade indicator (letters next to each other).
'wordless tune' becomes 'hum' (humming a tune).
'drum' after 'hum' is 'HUMDRUM'.
(Other definitions for humdrum that I've seen before include "Boringly monotonous" , "Dull (of existence)" , "Dull and uninteresting" , "Monotonous; commonplace" , "Unexciting" .)