Tail can't move a lot of water (8)
I believe the answer is:
atlantic
'a lot of water' is the definition.
'tail can't move' is the wordplay.
'move' indicates anagramming the letters.
'tail'+'cant'='tailcant'
'tailcant' is an anagram of 'ATLANTIC'.
(Other definitions for atlantic that I've seen before include "UK's nearest ocean" , "Big sea" , "Latin cat strangely in the sea" , "Tilt a can at the ocean" , "Great sea" .)