Investigator, note, heading for court (3)
I believe the answer is:
tec
'investigator' is the definition.
(short for 'detective')
'note heading for court' is the wordplay.
'note' becomes 'te' (**).
'heading for' suggests taking the first letters.
The initial letter of 'court' is 'c'.
'te'+'c'=''TEC'
(Other definitions for tec that I've seen before include "Busy" , "Investigator" , "Detective (colloq.)" , "CID officer (abbrev.)" .)