Shock of a tram crashing with unionist inside (6)
I believe the answer is:
trauma
'shock' is the definition.
(thesaurus)
'a tram crashing with unionist inside' is the wordplay.
'crashing with' indicates an anagram.
'unionist' becomes 'u' (abbreviation as in Democratic Unionist Party).
'inside' means one lot of letters goes inside another.
'tram' anagrammed with 'u' makes 'truma'.
'a' inserted within 'truma' is 'TRAUMA'.
'of' is the link.
(Other definitions for trauma that I've seen before include "hurt" , "Wounding shock" , "Psychological wound" , "Emotional shock producing lasting effect" , "Emotional upset" .)