A lot of water? Spot is heard (3)
I believe the answer is:
sea
'a lot of water?' is the definition.
(large body of water)
'spot is heard' is the wordplay.
'spot' becomes 'see' (I've seen this before).
'is heard' indicates a 'sounds like' (homophone) clue.
'see' is a homophone of 'SEA'.
(Other definitions for sea that I've seen before include "The tide" , "Part of ocean" , "Vast expanse" , "E.g. Baltic" , "The briny" .)