Its upper circle is boring! (6)
I believe the answer is:
tiring
'boring' is the definition.
(I've seen this before)
'its upper circle' is the wordplay.
'its' becomes 'it' (I've seen this in other clues).
'upper' says the letters should be written in reverse (I've seen 'upped' mean this (in down clue, made to go up)).
'circle' becomes 'ring' (synonyms).
'it' backwards is 'ti'.
'ti'+'ring'='TIRING'
'is' acts as a link.
(Other definitions for tiring that I've seen before include "Wearisome" , "Growing weary or bored" , "Exhausting or tedious" , "Becoming sleepy" , "Wearying, exhausting" .)