All in plate, very thin gruel (10)
I believe the answer is:
everything
'all' is the definition.
(all things)
'in plate very thin gruel' is the wordplay.
'in' says the answer is hidden in the clue.
'EVERYTHING' is hidden in the letters of 'plate very thin gruel'.
(Other definitions for everything that I've seen before include "All items" , "great deal" , "The whole lot" , "The whole shebang" , "the works" .)