Scold daughter about tiny plant that causes hay fever (7)
I believe the answer is:
ragweed
'plant that causes hay fever' is the definition.
'scold daughter about tiny' is the wordplay.
'scold' becomes 'rag' ('rag' can be a synonym of 'scold').
'daughter' becomes 'd'.
'about' means one lot of letters go next to another.
'tiny' becomes 'wee' (I've seen this before).
'd' put after 'wee' is 'weed'.
'rag'+'weed'='RAGWEED'
(Other definitions for ragweed that I've seen before include "Unwanted plant with yellow flowers" , "Plant of daisy family" , "Allergenic plant" , "Threat to horses" , "bloomer" .)