Account for what was once unembellished (7)
I believe the answer is:
explain
'account for' is the definition.
(I've seen this before)
'once unembellished' is the wordplay.
'once' becomes 'ex' (prefix meaning former or onetime).
'unembellished' becomes 'plain' (synonyms).
'ex'+'plain'='EXPLAIN'
'what was' is the link.
(Other definitions for explain that I've seen before include "Tell us how" , "Describe" , "Make understandable" , "Justify" , "Give reason for" .)