It's for play when rain interrupts the Test badly (5,3)
I believe the answer is:
train set
'it's for play' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'rain interrupts the test badly' is the wordplay.
'interrupts' is an insertion indicator.
'badly' indicates anagramming the letters (letters in the wrong order).
'test' anagrammed gives 'tset'.
'rain' placed into 'tset' is 'TRAIN-SET'.
'when' acts as a link.
Can you help me to learn more?
(Other definitions for train set that I've seen before include "scaled-down transport system" , "Toy for dads?" , "Strange treats in store for this model toy" , "little loco with this?" , "A child might play with this" .)