Instrument used thus to call on America (10)

'instrument' is the definition.
(sousaphone is a kind of instrument)

'thus to call on america' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'to call' becomes 'phone' ('phone' can be a synonym of 'call'**).
'on' is a charade indicator (letters next to each other).
'america' becomes 'usa' ('U.S.A.' can be a synonym of 'america').
'phone' put after 'usa' is 'usaphone'.
'so'+'usaphone'='SOUSAPHONE'

'used' is the link.