Rising whale and youngster emerge (4,2)
I believe the answer is:
crop up
'emerge' is the definition.
(thesaurus)
'rising whale and youngster' is the wordplay.
'rising' says the letters should be written backwards (going up in a down clue).
'whale' becomes 'orc' (term for a killer whale).
'and' is a charade indicator (letters next to each other).
'youngster' becomes 'pup' (I've seen this before).
'orc' reversed gives 'cro'.
'cro'+'pup'='CROP UP'
(Other definitions for crop up that I've seen before include "Occur by chance - like harvest?" , "Appear unexpectedly" , "Occur, arise" , "Occur unexpectedly - like harvest?" , "Happen" .)