Trappings showing majestic insignia on the outside (7)
I believe the answer is:
regalia
'trappings' is the definition.
(I have seen 'Trappings of royalty' mean 'regalia' so perhaps 'trappings' could also mean 'regalia')
'majestic insignia on the outside' is the wordplay.
'majestic' becomes 'regal' (synonyms).
'on the outside' means to remove the middle letters.
'insignia' with its centre taken out is 'ia'.
'regal'+'ia'='REGALIA'
'showing' is the link.
(Other definitions for regalia that I've seen before include "Orb and sceptre etc" , "Insignia of royalty or office" , "Royal trappings" , "Royal paraphernalia" , "Royal accoutrements" .)