Toothy smile after tea causes annoyance (7)
I believe the answer is:
chagrin
'annoyance' is the definition.
(I've seen this before)
'toothy smile after tea' is the wordplay.
'toothy smile' becomes 'grin' (I've seen this before).
'after' is a charade indicator (letters next to each other).
'tea' becomes 'cha' (cha is a type of tea).
'grin' after 'cha' is 'CHAGRIN'.
'causes' acts as a link.
(Other definitions for chagrin that I've seen before include "irritation?" , "anything but delight" , "Feeling of annoyance and disappointment" , "Disappointment, exasperation" , "Frustration at having failed" .)