Couple of heads of department under observation (3)
I believe the answer is:
duo
'couple' is the definition.
(two people)
'heads of department under observation' is the wordplay.
'heads of' suggests taking the first letters.
The first letters of 'department under observation' is 'DUO'.
'of' acts as a link.
(Other definitions for duo that I've seen before include "Couple, pair" , "Pair (of people, things)" , "Two of a kind" , "Two-person act" , "They might play" .)