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' indicates the central letters.
The central 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" .)