Remarkable person aboard Queen Elizabeth (4)
I believe the answer is:
oner
'remarkable person' is the definition.
(slang for a unique person or thing)
'aboard queen elizabeth' is the wordplay.
'aboard' becomes 'on'.
'queen elizabeth' becomes 'er' (abbreviation for Elizabeth Regina).
'on'+'er'='ONER'
(Other definitions for oner that I've seen before include "One pound note; big lie" , "Unique person" , "Big blow" , "expert" , "Remarkable thing" .)