Produces motives for us to appear in unusual case (6)
I believe the answer is:
causes
'produces motives' is the definition.
'causes' can be an answer for 'motives' (I've seen this in another clue). I am not certain of the 'produces' bit.
'us to appear in unusual case' is the wordplay.
'to appear in' means one lot of letters goes inside another.
'unusual' is an anagram indicator.
'case' with letters rearranged gives 'caes'.
'us' inserted inside 'caes' is 'CAUSES'.
'for' is the link.
Can you help me to learn more?
(Other definitions for causes that I've seen before include "'Reasons, grounds (6)'" , "Brings" , "Reasons why things happen" , "Origins or crusaders" , "They give rise to effects" .)