Inattentive worker catching disease (6)
I believe the answer is:
absent
'inattentive' is the definition.
(similar in meaning)
'worker catching disease' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'catching' means one lot of letters goes inside another.
'disease' becomes 'BSE' (bovine spongiform encephalopathy, mad cow disease).
'ant' enclosing 'bse' is 'ABSENT'.
(Other definitions for absent that I've seen before include "Forgetful, with mind on other things" , "Missing, lacking" , "Not present - vacuous" , "AWOL" , "Not where one should be" .)