Seeing union's leader boarding streetcar, a shock (6)
I believe the answer is:
trauma
'shock' is the definition.
(I've seen this before)
'seeing union's leader boarding streetcar a' is the wordplay.
'seeing union's leader' becomes 'u' (I can't justify this - if you can you should give a lot more credence to this answer).
'boarding' is an insertion indicator.
'streetcar' becomes 'tram' (synonyms).
'u' going inside 'tram' is 'traum'.
'traum'+'a'='TRAUMA'
Can you help me to learn more?
(Other definitions for trauma that I've seen before include "Severe shock, injury" , "Lasting shock or disturbance" , "Emotional damage" , "dreadful shock!" , "Physical or mental shock" .)