Bad reaction to colleague ringing work unit (7)
I believe the answer is:
allergy
'bad reaction' is the definition.
(allergic reaction)
'colleague ringing work unit' is the wordplay.
'colleague' becomes 'ally' (ally can mean a colleague or associate).
'ringing' indicates putting letters inside.
'work unit' becomes 'erg' (erg is a kind of work unit).
'ally' enclosing 'erg' is 'ALLERGY'.
'to' is the link.
(Other definitions for allergy that I've seen before include "Unfavourable reaction as to pollens or foods" , "Hypersensitive reaction of the body to a foreign substance" , "Adverse reaction (med.)" , "Abnormal reaction as to, say, pollen" , "unhealthy reaction" .)