Complete unknown covering front of robe with flounces (6)
I believe the answer is:
frilly
'with flounces' is the definition.
(having frills or flounces)
'complete unknown covering front of robe' is the wordplay.
'complete' becomes 'fill' (eg completing or filling in a form).
'unknown' becomes 'y' (algebra).
'covering' means one lot of letters goes inside another.
'front of' indicates taking the first letters.
The first letter of 'robe' is 'r'.
'fill'+'y'='filly'
'filly' enclosing 'r' is 'FRILLY'.
(Other definitions for frilly that I've seen before include "ruffled" , "Having flounces, ruffles, etc" , "Fancy" .)