Missing worker catches mad cow disease (6)
I believe the answer is:
absent
'missing' is the definition.
(similar in meaning)
'worker catches mad cow disease' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'catches' is an insertion indicator.
'mad cow disease' becomes 'bse' (bovine spongiform encephalopathy).
'ant' enclosing 'bse' is 'ABSENT'.
(Other definitions for absent that I've seen before include "lack of attention" , "Missing, lacking" , "Not attending" , "Inattentive" , "Away" .)