The sound of gruel being stirred at midnight (6)
I believe the answer is:
gurgle
'the sound' is the definition.
(gurgle is a kind of sound)
'gruel being stirred at midnight' is the wordplay.
'being stirred' indicates an anagram.
'at' is a charade indicator (letters next to each other) (I've seen this in other clues).
'midnight' becomes 'g' (the middle letter in 'night').
'gruel' anagrammed gives 'urgle'.
'urgle' put after 'g' is 'GURGLE'.
'of' acts as a link.
(Other definitions for gurgle that I've seen before include "drain sound" , "Hollow bubbling sound" , "Baby's noise" , "Lugger (anag)" , "Rodent snare" .)