Borders with white turnip and birch (4)
I believe the answer is:
whip
'birch' is the definition.
(thesaurus)
'borders with white turnip' is the wordplay.
'borders' means to remove the middle letters.
'white turnip' becomes 'ip' (I can't explain this - if you can you should give a lot more credence to this answer).
'with' with its centre removed is 'wh'.
'wh'+'ip'='WHIP'
'and' is the link.
Can you help me to learn more?
(Other definitions for whip that I've seen before include "Pudding" , "Scourge; steal" , "Legislator appointed by a party to enforce discipline" , "He maintains discipline among members" , "Party disciplinarian" .)