Tea on beam is an annoyance (7)
I believe the answer is:
chagrin
'an annoyance' is the definition.
(I've seen this before)
'tea on beam' is the wordplay.
'tea' becomes 'cha' (cha is a type of tea).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'beam' becomes 'grin' (I've seen this before).
'cha'+'grin'='CHAGRIN'
'is' is the link.
(Other definitions for chagrin that I've seen before include "Annoyance at having been humiliated" , "Arching with annoyed disappointment" , "Disappointment, exasperation" , "Emotion" , "Frustration at having failed" .)