Instrument dismissed husband used to stab Welsh buccaneer (5,5)
I believe the answer is:
mouth organ
'instrument' is the definition.
(mouth organ is a kind of instrument)
'dismissed husband used to stab welsh buccaneer' is the wordplay.
'dismissed' becomes 'out' (I've seen this before).
'husband' becomes 'h'.
'used to stab' is an insertion indicator (some letters stab a hole in a word).
'welsh buccaneer' becomes 'morgan' (privateer Henry Morgan).
'out'+'h'='outh'
'outh' going within 'morgan' is 'MOUTH-ORGAN'.
(Other definitions for mouth organ that I've seen before include "I can make music" , "Harmonica" , "tongue?" , "What produces musical sounds" .)