Plaice finally eaten by sweet relative (5)
I believe the answer is:
niece
'relative' is the definition.
(type of relative)
'plaice finally eaten by sweet' is the wordplay.
'finally' says to take the final letters.
'eaten by' means one lot of letters goes inside another (inserted letters get eaten).
'sweet' becomes 'nice' (I've seen this in other clues).
The last letter of 'plaice' is 'e'.
'e' put inside 'nice' is 'NIECE'.
(Other definitions for niece that I've seen before include "Family relation" , "A sibling's daughter" , "Son's cousin" , "A sibling's child" , "Female relation" .)