Shame about Grammar School being such a dirty place! (6)
I believe the answer is:
pigsty
'being such a dirty place' is the definition.
(pigsty can mean somewhere dirty or messy)
'shame about grammar school' is the wordplay.
'shame' becomes 'pity' (synonyms).
'about' is an insertion indicator.
'grammar school' becomes 'GS' (abbreviation for grammar school).
'pity' enclosing 'gs' is 'PIGSTY'.
(Other definitions for pigsty that I've seen before include "Lamb neck-chop" , "Very dirty abode" , "messy situation" , "Porcine pen" , "Without boots" .)