Terrible bores when not drunk... (5)
I believe the answer is:
sober
'not drunk' is the definition.
(I've seen this in another clue)
'terrible bores' is the wordplay.
'terrible' indicates anagramming the letters.
'bores' is an anagram of 'SOBER'.
'when' acts as a link.
(Other definitions for sober that I've seen before include "Abstemious in robes" , "Earnest" , "Serious, thoughtful" , "Unintoxicated" , "Sensible - clear-headed" .)