Business chief pockets £50 in big fiddle? (5)
I believe the answer is:
cello
'big fiddle?' is the definition.
(large fiddle-like instrument)
'business chief pockets ps50' is the wordplay.
'business chief' becomes 'CEO' (chief executive officer).
'pockets' is an insertion indicator.
'ps50' becomes 'll' (I am not sure about this - if you are sure you should believe this answer much more).
'ceo' placed around 'll' is 'CELLO'.
'in' acts as a link.
(Other definitions for cello that I've seen before include "big fiddle?" , "String quartet instrument" , "Bass stringed instrument" , "Tortelier's instrument" , "Bowed musical instrument" .)