Brother's daughter is said to be in French resort (5)
I believe the answer is:
niece
'brother's daughter' is the definition.
(I've seen this in another clue)
'said to be in french resort' is the wordplay.
'said to be' becomes 'e' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'french resort' becomes 'nice' (French coastal resort).
'e' placed inside 'nice' is 'NIECE'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for niece that I've seen before include "Preparation for curdling milk" , "Criminal" , "[PRIEST'S] daughter?" , "Brother's daughter" , "Family relation" .)