Trial involving long case (3,5)
I believe the answer is:
'case' is the definition.
'trial involving long' is the wordplay.
'trial' becomes 'test' (synonyms).
'involving' is an insertion indicator.
'long' becomes 'ache' (synonyms).
'test' placed around 'ache' is 'TEA-CHEST'.
(Other definitions for tea chest that I've seen before include "Metal lined container" , "Item famously thrown overboard" , "Crate (much used in moving house)" , "Leaves boxing" , "box of gunpowder, say" .)