First note in tonic sol-fa (3)
I believe the answer is:
doh
'first note in tonic sol-fa' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for doh that I've seen before include "Stupid me!" , "for 7 [HOMER] of course!" , "First note of major scale in music" , "Tonic (mus.)" , "First and eighth note in tonic sol-fa" .)