Claim new clothes start to reduce cause of bad reaction (8)
I believe the answer is:
allergen
'cause of bad reaction' is the definition.
(I've seen this before)
'claim new clothes start to reduce' is the wordplay.
'claim' becomes 'allege' (I've seen this before).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'clothes' indicates putting letters inside.
'start to' suggests taking the first letters.
The initial letter of 'reduce' is 'r'.
'allege'+'n'='allegen'
'allegen' enclosing 'r' is 'ALLERGEN'.
(Other definitions for allergen that I've seen before include "Substance inducing negative reaction" , "Substance triggering an abnormal bodily reaction" , "Protein" , "Substance to which body reacts" , "thing that causes a reaction" .)