Separate irritation, scratching head (5)
I believe the answer is:
other
'separate' is the definition.
(associated in meaning)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for other that I've seen before include "Different, alternative" , "Different, disparate" , "Alternative of two" , "Different; alternate" , "Different (from those mentioned)" .)