Follow an exceptional trial (5)
I believe the answer is:
trail
'follow' is the definition.
(trail can mean to follow in secret)
'an exceptional trial' is the wordplay.
'an exceptional' indicates an anagram (an unusual or exceptional ordering of the letters).
'trial' anagrammed gives 'TRAIL'.
(Other definitions for trail that I've seen before include "Be drawn belong behind" , "Drag along the ground" , "Be losing; track" , "Need to catch up with" , "Dog - track" .)