Women's backsides in border unnerve and frighten bird (4)
I believe the answer is:
wren
'bird' is the definition.
(wren is a kind of bird)
'women's backsides in border unnerve and frighten' is the wordplay.
I cannot quite see how this works, but
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
'women' could be 'w' and 'w' is located in the answer.
The remaining letters 're' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for wren that I've seen before include "17th Century architect" , "Bird - architect, 1632-1723" , "Very small brown songbird with erect tail" , "flier" , "Small singer" .)