School physics unit facing ruin (7)
I believe the answer is:
grammar
'school' is the definition.
(grammar school is a type of school)
'physics unit facing ruin' is the wordplay.
'physics unit' becomes 'gram' (gram is a unit of mass).
'facing' means one lot of letters go next to another.
'ruin' becomes 'mar' (I've seen this before).
'gram'+'mar'='GRAMMAR'
(Other definitions for grammar that I've seen before include "type of school" , "Syntax etc" , "Study of word and sentences" , "Study of words and sentences" , "Study of basic linguistic units" .)