Pass a hothead a note (3)
I believe the answer is:
doh
'a note' is the definition.
(alternative spelling of 'do' as in do re mi)
'pass a hothead' is the wordplay.
'pass' becomes 'do' (doing is a kind of passing).
'a hothead' becomes 'h' (head letter of 'hot').
'do'+'h'='DOH'
(Other definitions for doh that I've seen before include "Stupid me!" , "The note C" , "whoops!" , "for 7 [HOMER] of course!" , "Homeric grunt of annoyance" .)