Deity repeatedly upset another — that's very unusual (4,4)
I believe the answer is:
rara avis
'very unusual' is the definition.
(I've seen this before)
'deity repeatedly upset another' is the wordplay.
I cannot quite understand how this works, but
'deity' could be 'ra' (Ra is an example) and 'ra' is found in the answer.
'another' could be 'r' (I've seen this in other clues) and 'r' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for rara avis that I've seen before include "Uncommon person/thing" , "curiosity" , "An unusual person, or bird" , "An uncommon thing" , "strange bird" .)