Trial involving long case (3,5)
I believe the answer is:
tea chest
'case' is the definition.
(thesaurus)
'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" .)