Boss must carry doubly large instrument (5)
I believe the answer is:
cello
'instrument' is the definition.
(cello is a kind of instrument)
'boss must carry doubly large' is the wordplay.
'boss' becomes 'CEO' (chief executive officer).
'must carry' is an insertion indicator (I've seen this in other clues).
'doubly' shows that letters should be duplicated.
'large' becomes 'L' (eg in clothes sizes).
'l' doubled is 'll'.
'ceo' going around 'll' is 'CELLO'.
(Other definitions for cello that I've seen before include "Musical instrument like large violin held between knees" , "Music-maker" , "Lloyd Webber's instrument" , "Played by Rostropovich" , "Bowed musical instrument" .)