Verity's inside a trench (3)
I believe the answer is:
rut
'a trench' is the definition.
(rut is a kind of trench)
'verity's inside' is the wordplay.
'verity' becomes 'truth' (synonyms).
'inside' means to look at the middle letters.
The middle letters of 'truth' are 'RUT'.
(Other definitions for rut that I've seen before include "Mating period for deer" , "Dull routine behaviour" , "Undeviating routine" , "Deep track in soft ground" , "Groove made by wheel" .)