The aforementioned heads of department interrupted trade talks occasionally (5)
I believe the answer is:
ditto
'the aforementioned' is the definition.
'heads of department interrupted trade talks occasionally' is the wordplay.
'heads of' says to take the initial letters.
The first letters of 'department interrupted trade talks occasionally' is 'DITTO'.
(Other definitions for ditto that I've seen before include "Copy" , "Two small marks indicating the same again" , "Mark used to indicate repetition" , "Likewise" , "Repeat the word above" .)