They are why there’s a U in “luggage” (6)
I believe the answer is:
causes
'they are why' is the definition.
The definition suggests an adverb but the answer is not.
'a u in luggage' is the wordplay.
'in' indicates putting letters inside.
'luggage' becomes 'cases' (I've seen this before).
'u' going within 'cases' is 'CAUSES'.
'there's' is the link.
Can you help me to learn more?
(Other definitions for causes that I've seen before include "Reasons why things happen" , "They bring about effects" , "See 5" , "Origins or crusaders" , "'Reasons, grounds (6)'" .)