Coach with roof off, wet (4)
I believe the answer is:
rain
'wet' is the definition.
(wet weather)
'coach with roof off' is the wordplay.
'coach' becomes 'train' (both can mean to instruct a sportsperson).
'with roof off' means to remove the first letter (roof means the letter at the top).
'train' with its first letter removed is 'RAIN'.
(Other definitions for rain that I've seen before include "Pelt" , "Come down in showers" , "The ---, it ---eth every day (T.Night)" , "See 12" , "previous rule?" .)