Harmony in a musical instrument (10)

I believe the answer is:

concertina

'musical instrument' is the definition.
(concertina is a kind of musical instrument)

'harmony in a' is the wordplay.
'harmony' becomes 'concert' (I've seen this in another clue).
'concert'+'in'+'a'='CONCERTINA'

(Other definitions for concertina that I've seen before include "Squeeze box" , "Squeeze-box" , "Collapse" , "Instrument played like an accordion" , "gig belonging to American" .)

I've seen this clue in The Guardian, The Globe and Mail, The Irish Independent and The Telegraph.

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