Expression of approval: praise set in stone (7)
I believe the answer is:
plaudit
'expression of approval' is the definition.
(I've seen this before)
'praise set in stone' is the wordplay.
'praise' becomes 'laud' (lauding is a kind of praising).
'set in' means one lot of letters goes inside another.
'stone' becomes 'pit' (synonyms).
'laud' going inside 'pit' is 'PLAUDIT'.
(Other definitions for plaudit that I've seen before include "Salute" , "Cheer" , "Enthusiastic approval" , "Praise, encomium" , "Round of applause" .)