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