Spooner's card game ruined, proving to be a trial (5,3)
I believe the answer is:
dummy run
'a trial' is the definition.
(I've seen this before)
'spooner's card game ruined proving' is the wordplay.
I cannot quite see how this works, but
'game' could be 'ru' (rugby union) and 'ru' is found within the answer.
This explanation may well be incorrect...
'to be' acts as a link.
Can you help me to learn more?
(Other definitions for dummy run that I've seen before include "Practice session" , "Rehearsal" , "A try-out or test" , "Practice, trial" .)