Was jumper found in His Majesty's loo? (8)
I believe the answer is:
washroom
'loo?' is the definition.
(I know that loo can be written as washroom)
'was jumper found in his majesty's' is the wordplay.
'jumper' becomes 'roo' (short for kangaroo).
'found in' indicates putting letters inside.
'his majesty' becomes 'hm' (short for His Majesty).
'roo' placed into 'hm' is 'hroom'.
'was'+'hroom'='WASHROOM'
(Other definitions for washroom that I've seen before include "Where Americans go" , "Clean up place" , "bog across the pond" , "US loo" .)