'Do it!' I snapped, terribly frustrated? (12)
I believe the answer is:
disappointed
'frustrated?' is the definition.
(disappointing is a kind of frustrating)
'do it i snapped terribly' is the wordplay.
'terribly' is an anagram indicator.
'do'+'it'+'i'+'snapped'='doitisnapped'
'doitisnapped' is an anagram of 'DISAPPOINTED'.
(Other definitions for disappointed that I've seen before include "Unhappy" , "Saddened" , "Sad and let-down" , "Let-down, frustrated" , "Feeling sad" .)