Breather from running nullified on reflection (4)
I believe the answer is:
lung
'breather' is the definition.
(I've seen this before)
'from running nullified on reflection' is the wordplay.
'from' indicates the answer is hidden within the clue.
'on reflection' shows that the letters should be reversed in order.
'gnul' is hidden within 'running nullified'.
'gnul' back-to-front is 'LUNG'.
(Other definitions for lung that I've seen before include "breather" , "Organ used for breathing" , "Of sound mind" , "Pulmonary organ" , "Respiratory organ" .)