Celebration, nervy, as Iran is involved (11)
I believe the answer is:
anniversary
'celebration' is the definition.
(I've seen this before)
'nervy as iran is involved' is the wordplay.
'is involved' indicates anagramming the letters (involved can mean confusing or complex).
'nervy'+'as'+'iran'='nervyasiran'
'nervyasiran' anagrammed gives 'ANNIVERSARY'.
(Other definitions for anniversary that I've seen before include "Celebration" , "Eg, a birthday" , "Day marking a past event" , "Yearly commemoration" , "special occasion" .)