Worker admits messing about in task (10)
I believe the answer is:
assignment
'task' is the definition.
(thesaurus)
'worker admits messing about' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'admits' is an insertion indicator.
'about' is an anagram indicator.
'messing' with letters rearranged gives 'ssignme'.
'ant' placed around 'ssignme' is 'ASSIGNMENT'.
'in' is the link.
(Other definitions for assignment that I've seen before include "Task, piece of work" , "Duty to be done" , "job" , "Task or duty" .)