Very small, not starting to be irritating (5)
I believe the answer is:
itchy
'be irritating' is the definition.
(I've seen this before)
'very small not starting' is the wordplay.
'very small' becomes 'titchy' (I've seen this before).
'not starting' means to remove the first letter.
'titchy' with its initial letter removed is 'ITCHY'.
'to' acts as a link.
(Other definitions for itchy that I've seen before include "Fidgety" , "Unable to relax" , "Irritating to the skin" , "Tickling" , "Have . . . . . feet - be restless, want to travel" .)