Most of journey with a learner will be an ordeal (5)
'an ordeal' is the definition.
(I've seen this before)
'most of journey with a learner' is the wordplay.
'most of' means to remove the last letter.
'journey' becomes 'trip' (trip is a kind of journey).
'with' means one lot of letters go next to another.
'learner' becomes 'l'.
'trip' with its last letter taken away is 'tri'.
'will be' is the link.
(Other definitions for trial that I've seen before include "French actor who sings", "Test of performance", "Hearing - difficulty", "Ordeal, perhaps in court", "Court proceeding".)