Casual partner extremely upset, seeing what sister sometimes wears (8)
I believe the answer is:
scapular
'what sister sometimes wears' is the definition.
I can't judge whether this defines the answer.
'casual partner extremely upset' is the wordplay.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'upset' is an anagram indicator.
'partner' with its middle taken out is 'pr'.
'casual'+'pr'='casualpr'
'casualpr' with letters rearranged gives 'SCAPULAR'.
'seeing' acts as a link.
Can you help me to learn more?
(Other definitions for scapular that I've seen before include "Of the shoulder blades - plus a car (anag)" , "Relating to the shoulder blade" , "Of the religious garment worn over the shoulders" , "Rascal up to get monk's short cloak" .)