Practise heading off downpour (4)
I believe the answer is:
rain
'downpour' is the definition.
(thesaurus)
'practise heading off' is the wordplay.
'practise' becomes 'train'.
'heading off' means to remove the first letter.
'train' with its first letter taken away is 'RAIN'.
(Other definitions for rain that I've seen before include "Fall in large quantities" , "Wet weather" , "Storm" , "Precipitation" , "eg A shower" .)