Brass section's top piercing note (4)
I believe the answer is:
dosh
'brass' is the definition.
(both are slang terms for money)
'section's top piercing note' is the wordplay.
'top' suggests taking the first letters.
'piercing' means one lot of letters goes inside another (some letters pierce a hole in others).
'note' becomes 'doh' (alternative spelling of 'do' as in do re mi**).
The first letter of 'sections' is 's'.
's' placed within 'doh' is 'DOSH'.
(Other definitions for dosh that I've seen before include "Bread - gravy - dough" , "Money (sl.)" , "Moolah (slang)" , "Spondulicks" , "cash" .)