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' says to take the initial letters.
The initial letters of 'department under observation' is 'DUO'.
'of' is the 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" .)