Upper-class, boarding public transport? A shock (6)
I believe the answer is:
trauma
'shock' is the definition.
(thesaurus)
'upper-class boarding public transport? a' is the wordplay.
'upper-class' becomes 'u' ('u' can be similar in meaning to 'upper-class').
'boarding' indicates putting letters inside.
'public transport?' becomes 'tram' (I've seen this before).
'u' put inside 'tram' is 'traum'.
'traum'+'a'='TRAUMA'
(Other definitions for trauma that I've seen before include "Deeply distressing or disturbing experience" , "hurt" , "Emotional shock producing lasting effect" , "Shock to the body or mind" , "Physical injury - powerful emotional shock" .)