River in children's game is one mentioned in Bible (6)
I believe the answer is:
tigris
'river' is the definition.
(I know that Tigris is a river)
'children's game is one mentioned in bible' is the wordplay.
'children's game' becomes 'tig'.
'is one' becomes 'i' (Roman numeral).
'mentioned in' is an insertion indicator.
'bible' becomes 'rs' (I can't justify this - if you can you should believe this answer much more).
'i' inserted within 'rs' is 'ris'.
'tig'+'ris'='TIGRIS'
'in' is the link.
Can you help me to learn more?
(Other definitions for tigris that I've seen before include "It flows through Iraq" , "Great river of 30 across" , "Grit is found in great river of Iraq" , "Great river flowing from Turkey through Iraq" , "flower of Iraq" .)