Confederate arresting work unit causes a reaction (7)
I believe the answer is:
allergy
'reaction' is the definition.
(I've seen this before)
'confederate arresting work unit' is the wordplay.
'confederate' becomes 'ally' (synonyms).
'arresting' means one lot of letters goes inside another.
'work unit' becomes 'erg' (erg is a kind of work unit).
'ally' enclosing 'erg' is 'ALLERGY'.
'causes a' is the link.
(Other definitions for allergy that I've seen before include "Unfavourable reaction to pollen, certain foods etc" , "Antipathy (colloq.)" , "Cause of ill-feeling" , "Susceptibility" , "Producer of Hay Fever" .)