Tons come down to exercise (5)
I believe the answer is:
train
'exercise' is the definition.
(training is a kind of exercising)
'tons come down' is the wordplay.
'tons' becomes 't'.
'come down' becomes 'rain' (raining is a kind of coming down).
't'+'rain'='TRAIN'
'to' is the link.
(Other definitions for train that I've seen before include "union's lengthy item" , "Number of animals moving in a line, like camels" , "Rail service" , "Retinue; sequence" , "Do exercises" .)