Exclamation: go quietly! (4)
I believe the answer is:
gosh
'exclamation' is the definition.
(I've seen this in another clue)
'go quietly' is the wordplay.
'quietly' becomes 'sh' (both are used to tell someone to be quiet).
'go'+'sh'='GOSH'
(Other definitions for gosh that I've seen before include "Good heavens!" , "Well I never!" , "Fancy that!" , "Crikey" , "Exclamation of surprise" .)