Worker, carrying infectious disease, not present (6)
I believe the answer is:
absent
'not present' is the definition.
('absent' is the opposite to 'present')
'worker carrying infectious disease' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'carrying' is an insertion indicator.
'infectious disease' becomes 'bse' (bovine spongiform encephalopathy, mad cow disease).
'ant' going around 'bse' is 'ABSENT'.
(Other definitions for absent that I've seen before include "Not at a specified place" , "nowhere to be seen" , "elsewhere" , "Not where one should be" , "Inattentive" .)