I'm so upset about the Rolls having possibly Minor significance! (6)
I believe the answer is:
morris
'possibly minor significance' is the definition.
The definition suggests a singular noun which matches the answer.
'i'm so upset about the rolls' is the wordplay.
'upset' indicates anagramming the letters.
'about' is an insertion indicator.
'the rolls' becomes 'RR' (Rolls Royce).
'im'+'so'='imso'
'imso' with letters rearranged gives 'mois'.
'mois' going around 'rr' is 'MORRIS'.
'having' acts as a link.
Can you help me to learn more?
(Other definitions for morris that I've seen before include "William --, English designer and novelist" , "socialist craftsman" , "Traditional English dance" , "English folk dance performed by men in costume" , "19th-century designer" .)