Rail not bent being tested (2,5)
I believe the answer is:
on trial
'being tested' is the definition.
(I've seen this in another clue)
'rail not bent' is the wordplay.
'bent' indicates an anagram.
'rail'+'not'='railnot'
'railnot' with letters rearranged gives 'ON TRIAL'.
(Other definitions for on trial that I've seen before include "Undergoing scrutiny" , "Facing action" , "In court to answer charges" , "Subject to being found satisfactory" , "heard?" .)