Reprimand a good-looker adopting flash name (8)
I believe the answer is:
admonish
'reprimand' is the definition.
(I've seen this before)
'a good-looker adopting flash name' is the wordplay.
'good looker' becomes 'dish' (I've seen this in other clues).
'adopting' means one lot of letters goes inside another (inserted letters are taken on or adopted).
'flash' becomes 'mo' (both mean a very short period of time).
'name' becomes 'n' (common abbreviation e.g. on forms).
'mo'+'n'='mon'
'dish' going around 'mon' is 'dmonish'.
'a'+'dmonish'='ADMONISH'
(Other definitions for admonish that I've seen before include "censure" , "Warn sternly" , "Gently reprove" , "Forewarn" , "Mildly reprove" .)