Cavil at Tube-rail returns showing invalidation (8)
I believe the answer is:
rebuttal
'invalidation' is the definition.
(invalidating or rebutting something)
'cavil at tube-rail returns showing' is the wordplay.
'showing' says the answer is hidden in the clue (the answer is shown in the clue).
'returns' shows that the letters should be reversed in order.
'lattuber' can be found hidden inside 'cavil at tube rail'.
'lattuber' backwards is 'REBUTTAL'.
(Other definitions for rebuttal that I've seen before include "Counter-argument" , "A contradiction of evidence" , "statement delivered to counter" , "Confutation" , "Answer" .)