At first, cinemagoer natters rudely during tear-jerker, causing dismay (13)
'dismay' is the definition.
'at first cinemagoer natters rudely during tear-jerker' is the wordplay.
'at' means one lot of letters go next to another.
'first cinemagoer' becomes 'c' (1st letter of 'cinemagoer').
'rudely' indicates an anagram.
'during' is an insertion indicator.
'tear-jerker' becomes 'onion' (cutting onions can make you cry).
'natters' with letters rearranged gives 'sternat'.
'sternat' placed into 'onion' is 'onsternation'.
'c'+'onsternation' is 'CONSTERNATION'.
'causing' acts as a link.
(Other definitions for consternation that I've seen before include "Feelings of dismay", "Worry", "Dismay - cast-iron tenon (anag)", "Anxiety, dismay", "Alarm".)