Drink is terribly quiet (3)
I believe the answer is:
sip
'drink' is the definition.
(I know that sip is a kind of drink)
'is terribly quiet' is the wordplay.
'terribly' is an anagram indicator.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'is' with letters rearranged gives 'si'.
'si'+'p'='SIP'
(Other definitions for sip that I've seen before include "Smallest drink" , "Thimbleful" , "Drink, small mouthful" , "Drink slowly" , "Drink gingerly" .)