Toy in test I ran, failing (5,3)
I believe the answer is:
train set
'toy' is the definition.
(train set is a type of toy)
'test i ran failing' is the wordplay.
'failing' indicates an anagram (a failed spelling of the word).
'test'+'i'+'ran'='testiran'
'testiran' with letters rearranged gives 'TRAIN-SET'.
'in' acts as a link.
(Other definitions for train set that I've seen before include "Toy railway" , "little loco with this?" , "A child might play with this" , "Strange treats in store for this model toy" , "Nitrates (anag) - Hornby product?" .)