Inside word on understanding trial (6)
I believe the answer is:
ordeal
'trial' is the definition.
(I know that ordeal is a type of trial)
'inside word on understanding' is the wordplay.
'inside' says to take the centre.
'on' says to put letters next to each other.
'understanding' becomes 'deal' (deal is a kind of understanding).
The centre of 'word' is 'or'.
'or'+'deal'='ORDEAL'
(Other definitions for ordeal that I've seen before include "A tough time in Laredo" , "Torment" , "Difficult experience that tests endurance" , "Unpleasant and prolonged experience" , "Trial; nightmare" .)