To sail over river in heavy mist is child's play if you can get over it
I believe the answer is:
leapfrog
'child's play if you can get over it' is the definition.
I can't judge whether this defines the answer.
'to sail over river in heavy mist' is the wordplay.
'to sail over' becomes 'leap' (I can't justify this - if you can you should give a lot more credence to this answer).
'river' becomes 'r'.
'in' means one lot of letters goes inside another.
'heavy mist' becomes 'fog' (I've seen this before).
'r' placed into 'fog' is 'frog'.
'leap'+'frog'='LEAPFROG'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for leapfrog that I've seen before include "Vaulting game" , "Game in which one child jumps over another" , "Advance by jumping over the opposition" , "Overtake in position" , "Child's game of jumping over competitors" .)