Temperature dropped in hot water layer (4)
I believe the answer is:
seam
'layer' is the definition.
(seam is a kind of layer)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for seam that I've seen before include "Stratum (of a mineral)" , "Join round cricket ball" , "The line where two edges are stitched together" , "Sewn join" , "Underground layer of mineral" .)