Their sharp points scratch this, at intervals? (5)
I believe the answer is:
cacti
'their sharp points' is the definition.
I can't judge whether this defines the answer.
'scratch this at intervals?' is the wordplay.
'at intervals?' indicates alternate letters (letters are taken at regular intervals).
'scratch'+'this'='scratchthis'
The alternate letters of 'scratchthis' are 'CACTI'.
(Other definitions for cacti that I've seen before include "tuna, for example" , "Plants native to arid regions" , "Tunas" , "Plants of dry regions" , "flowers" .)