Plant starts to ruin everything in border (7)
I believe the answer is:
skirret
'plant' is the definition.
(skirret is a kind of plant)
'starts to ruin everything in border' is the wordplay.
'starts to' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'border' becomes 'skirt' (skirting is a kind of bordering**).
The initial letters of 'ruin everything' is 're'.
're' going within 'skirt' is 'SKIRRET'.
(Other definitions for skirret that I've seen before include "Plant of parsley family with edible root; striker (anag.)" , "tubers" .)