Note about trough (3)
I believe the answer is:
hod
'trough' is the definition.
(a hod is a trough used by a builder)
'note about' is the wordplay.
'note' becomes 'doh' (alternative spelling of 'do' as in do re mi).
'about' says the letters should be written backwards.
'doh' reversed gives 'HOD'.
(Other definitions for hod that I've seen before include "Open trough for carrying bricks" , "Carrier for bricks" , "Brick-carrier's holder" , "A bricklayer carries bricks on it" , "Carrier for a bricklayer" .)