Irritant for a princess (3)
I believe the answer is:
pea
'irritant for a princess' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pea that I've seen before include "Cause of a princess's insomnia?" , "It grows in 23 Down" , "Tiny stone" , "12 in a pod" , "Spherical vegetable" .)