Couldn't find the reason for unsuccessful lawsuit (4,5)
I believe the answer is:
lost cause
'unsuccessful lawsuit' is the definition.
The definition suggests a singular noun which matches the answer.
'couldn't find the reason' is the wordplay.
'couldn't find' becomes 'lost' (I am not sure about this - if you are sure you should believe this answer much more).
'the reason' becomes 'cause' (I've seen this before).
'lost'+'cause'='LOST CAUSE'
'for' is the link.
Can you help me to learn more?
(Other definitions for lost cause that I've seen before include "you won't influence him" , "write-off" , "Hopeless endeavour" , "Hopeless campaign" , "patronised by Jude?" .)