Disconcerted by return of leading ticket seller (3,3)
I believe the answer is:
put out
'disconcerted' is the definition.
(thesaurus)
'by return of leading ticket seller' is the wordplay.
'by' says to put letters next to each other.
'return of' is a reversal indicator.
'leading' becomes 'up' ('up' can be similar in meaning to 'leading').
'ticket seller' becomes 'tout' (I've seen this before).
'up' in reverse letter order is 'pu'.
'pu' next to 'tout' is 'PUT-OUT'.
(Other definitions for put out that I've seen before include "Publish -- annoy" , "Broadcast; inconvenienced" , "Extinguished" , "expelled" , "Broadcast - angered" .)