Trembling finger, finally on the buzzer! (6)
I believe the answer is:
hornet
'buzzer' is the definition.
(something that buzzes)
'trembling finger finally on the' is the wordplay.
'trembling' is an anagram indicator.
'finally' indicates one should take the final letters.
The final letter of 'finger' is 'r'.
'r'+'on'+'the'='ronthe'
'ronthe' anagrammed gives 'HORNET'.
(Other definitions for hornet that I've seen before include "Large stinging wasp" , "Stingy type" , "Throne for a kind of wasp" , "Small flier" , "A large wasp with serious sting" .)