Excellent advice corporation rejected (6)
I believe the answer is:
tiptop
'excellent' is the definition.
(I've seen this before)
'advice corporation rejected' is the wordplay.
'advice' becomes 'tip' (I've seen this before).
'corporation' becomes 'pot' ('pot' can be a synonym of 'corporation').
'rejected' shows that the letters should be reversed in order.
'pot' written backwards gives 'top'.
'tip'+'top'='TIPTOP'
(Other definitions for tiptop that I've seen before include "Best of its kind" , "Jolly good, excellent" , "First-rate" , "Informally excellent, very best" , "The height of excellence" .)