Diagnose problem that expressed mental stress (8)
I believe the answer is:
agonised
'expressed mental stress' is the definition.
(agony can mean mental stress)
'diagnose problem' is the wordplay.
'problem' indicates anagramming the letters.
'diagnose' with letters rearranged gives 'AGONISED'.
'that' is the link.
(Other definitions for agonised that I've seen before include "Suffered mental anguish" , "Very pained" , "experienced pain" , "Worried greatly" , "Felt mental anguish (over a decision)" .)