Grand joke by one interrupting Scotsman (8)
I believe the answer is:
majestic
'grand' is the definition.
(thesaurus)
'joke by one interrupting scotsman' is the wordplay.
'joke' becomes 'jest' ('jest' can be a synonym of 'joke'**).
'by' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'interrupting' is an insertion indicator.
'scotsman' becomes 'mac'.
'jest'+'i'='jesti'
'jesti' going within 'mac' is 'MAJESTIC'.
(Other definitions for majestic that I've seen before include "Grand, impressive" , "Of royal appearance or status" , "Regal - stately" , "Regal, imperial" , "Stately; sublime" .)