Sample of brownie certified by family member (5)
I believe the answer is:
Here is my best explanation:
'family member' is the definition.
(I've seen this before)
'sample of brownie certified by' is the subsidiary indication.
'sample of' indicates a hidden word (I've seen this in other clues).
'niece' is hidden in the letters of 'brownie certified by'.
(Other definitions for niece that I've seen before include "Brother's daughter", "Girl", "girl further down the line", "A young female relative", "member of different generation", "A sibling's child", "Daughter of a sibling", "One of the family", "Sister's daughter" and "Female family member".)