Corrosive cacti sent round America (7)
I believe the answer is:
caustic
'corrosive' is the definition.
(I know that corrosive can be written as caustic)
'cacti sent round america' is the wordplay.
'sent' indicates anagramming the letters (send can mean to excite or stimulate).
'round' is an insertion indicator.
'america' becomes 'US' (United States).
'cacti' anagrammed gives 'catic'.
'catic' placed around 'us' is 'CAUSTIC'.
(Other definitions for caustic that I've seen before include "Severe" , "Burning, corroding" , "Harsh or corrosive" , "Burning, corrosive" , "Acrimonious - sarcastic" .)