Shock as naked guy boards vehicle close to arena (6)
I believe the answer is:
trauma
'shock' is the definition.
(thesaurus)
'naked guy boards vehicle close to arena' is the wordplay.
'naked' indicates the central letters.
'boards' means one lot of letters goes inside another.
'vehicle' becomes 'tram' (I've seen this before**).
'close to' suggests the final letters (the close/ending of).
The middle letter of 'guy' is 'u'.
The final letter of 'arena' is 'a'.
'u' placed within 'tram' is 'traum'.
'traum'+'a'='TRAUMA'
'as' acts as a link.
(Other definitions for trauma that I've seen before include "Deeply distressing or disturbing experience" , "Deep distress; injury" , "Severe shock, injury" , "Bodily or mental injury" , "Psychological 23" .)