Seat in empty gallery likely to snap? (6)
I believe the answer is:
grumpy
'likely to snap?' is the definition.
(I've seen this before)
'seat in empty gallery' is the wordplay.
'seat' becomes 'rump' (synonyms).
'in' indicates putting letters inside.
'empty' says to hollow out the word (remove centre letters).
'gallery' with its middle removed is 'gy'.
'rump' going into 'gy' is 'GRUMPY'.
(Other definitions for grumpy that I've seen before include "Doc's irritable pal" , "Annoyed and irritable" , "Gloomy and irritable like sour old man" , "Black-faced" , "An ill-tempered dwarf of group of Seven" .)