Big wave -- something in the sea that is ridden precariously? (6,5)
I believe the answer is:
roller skate
'that is ridden precariously?' is the definition.
'big wave something in the sea' is the wordplay.
'big wave' becomes 'roller'.
'something in the sea' becomes 'skate' (type of fish).
'roller'+'skate'='ROLLER-SKATE'
(Other definitions for roller skate that I've seen before include "Wheeled boot" , "Go on wheeled shoes" , "Heavy footwear" , "this on your foot?" , "one making child move fast?" .)