Lots about reason, but it's no use (4,5)
I believe the answer is:
lost cause
'it's no use' is the definition.
I can't tell whether this defines the answer.
'lots about reason' is the wordplay.
'about' indicates an anagram.
'reason' becomes 'cause' (I've seen this in another clue).
'lots' is an anagram of 'lost'.
'lost'+'cause'='LOST CAUSE'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for lost cause that I've seen before include "Hopeless quest" , "Hopeless case" , "write-off" , "patronised by Jude?" , "Hopeless campaign" .)