Cuppa cold on hot train (5)
I believe the answer is:
teach
'train' is the definition.
(I've seen this in another clue)
'cuppa cold on hot' is the wordplay.
'cuppa' becomes 'tea' (informal term for a cup of tea).
'cold' becomes 'c' (eg on taps).
'on' means one lot of letters go next to another.
'hot' becomes 'h' (abbreviation eg on taps).
'tea'+'c'+'h'='TEACH'
(Other definitions for teach that I've seen before include "Cheat (anag)" , "Convey knowledge to students" , "train" , "Give instruction to" , "Educate 4 across" .)