Small operation involving new countenance (8)
I believe the answer is:
sanction
'countenance' is the definition.
(I know that countenance can be written as sanction)
'small operation involving new' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'operation' becomes 'action'.
'involving' indicates putting letters inside.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'action' going around 'n' is 'anction'.
's'+'anction'='SANCTION'
(Other definitions for sanction that I've seen before include "Agree to - penalty" , "Allow; punish" , "Allow; penalty" , "Official permission or authority" , "Permit - punishment for breach of treaty" .)