Seen in mirror, dainty frock (6)
I believe the answer is:
ordain
'frock' is the definition.
(thesaurus)
'seen in mirror dainty' is the wordplay.
'seen in' indicates a hidden word.
'ORDAIN' is hidden in the letters of 'mirror dainty'.
(Other definitions for ordain that I've seen before include "Decree or consecrate" , "Either prescribe" , "Invest with priestly authority" , "Make someone a clergyman" , "Rule" .)