Tops up hot water (4)
I believe the answer is:
spot
'hot water' is the definition.
(thesaurus)
'tops up' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'tops' reversed gives 'SPOT'.
(Other definitions for spot that I've seen before include "Small mark; see" , "Catch sight of - pimple" , "See; pimple" , "Blob" , "See - speckle" .)