Good rule to mix up thin porridge (5)
I believe the answer is:
gruel
'thin porridge' is the definition.
(I've seen this before)
'good rule to mix up' is the wordplay.
'good' becomes 'g' (abbreviation**).
'to mix up' is an anagram indicator.
'rule' is an anagram of 'ruel'.
'g'+'ruel'='GRUEL'
(Other definitions for gruel that I've seen before include "Unappetising meal of boiled oatmeal" , "Thin soup" , "Luger (anag)" , "Thin liquid food" , "Thin oatmeal dish" .)