Small rescue dog bites backside of Japanese emperor (8)

'emperor' is the definition.
(I've seen this in another clue)

'small rescue dog bites backside of japanese' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'rescue dog' becomes 'lassie'.
'bites' means one lot of letters goes inside another.
'backside of' suggests the final letters.
The last letter of 'japanese' is 'e'.
's'+'lassie'='slassie'
'slassie' going around 'e' is 'SELASSIE'.