Herbs beginning to cause rash (6)
I believe the answer is:
chives
'herbs' is the definition.
(I've seen this before)
'beginning to cause rash' is the wordplay.
'beginning to' suggests taking the first letters.
'rash' becomes 'hives' (hives is a kind of rash).
The initial letter of 'cause' is 'c'.
'c'+'hives'='CHIVES'
(Other definitions for chives that I've seen before include "Leaves used as onion-flavoured seasoning" , "Herbs with long tubular onionflavoured leaves" , "Plants" , "Plant of onion family" , "Herb of the onion family" .)