Task in which worker keeps messing about (10)
I believe the answer is:
assignment
'task' is the definition.
(thesaurus)
'worker keeps messing about' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'keeps' means one lot of letters goes inside another.
'about' indicates an anagram.
'messing' anagrammed gives 'ssignme'.
'ant' enclosing 'ssignme' is 'ASSIGNMENT'.
'in which' acts as a link.
(Other definitions for assignment that I've seen before include "Duty to be done" , "Task or duty" , "job" , "Task, piece of work" .)