Experiment involving long container (3,5)
I believe the answer is:
tea chest
'container' is the definition.
(tea chest is a kind of container)
'experiment involving long' is the wordplay.
'experiment' becomes 'test' (test is a kind of experiment).
'involving' is an insertion indicator.
'long' becomes 'ache' (to long for something is to ache for it).
'test' enclosing 'ache' is 'TEA-CHEST'.
(Other definitions for tea chest that I've seen before include "case for move" , "box exported from India?" , "Foil-lined wooden box" , "box of gunpowder, say" , "Dry container" .)