Canoe floating in water (5)
I believe the answer is:
ocean
'water' is the definition.
(oceans are bodies of water)
'canoe floating' is the wordplay.
'floating' is an anagram indicator (float can mean fluctuate or oscillate).
'canoe' is an anagram of 'OCEAN'.
'in' acts as a link.
(Other definitions for ocean that I've seen before include "Huge sea" , "Very large area of sea" , "Indian perhaps" , "tons of water" , "Immense expanse or quantity" .)