Meitner and Hahn's first fantastic instrument (8)

'instrument' is the definition.
(I know that theremin is a type of electronic instrument)

'meitner and hahn's first fantastic' is the wordplay.
'and' says to put letters next to each other.
'hahn's first' becomes 'h' (1st letter of 'hahn').
'fantastic' indicates anagramming the letters (a fantastic or improbable ordering of the letters).
'meitner' put after 'h' is 'hmeitner'.
'hmeitner' is an anagram of 'THEREMIN'.