Noah's second boy turned out to be one riding elephant (6)
I believe the answer is:
mahout
'one riding elephant' is the definition.
(I know that mahouts ride elephants)
'noah's second boy turned out' is the wordplay.
'noah's second boy' becomes 'Ham' (Ham was Noah's second son).
'turned' says the letters should be written in reverse.
'ham' reversed gives 'mah'.
'mah'+'out'='MAHOUT'
'to be' is the link.
(Other definitions for mahout that I've seen before include "Jumbo rider" , "Elephant keeper and driver" , "Elephant handler" , "Keeper and driver of elephants" , "Elephant-driver" .)