Confounded plant closing borders, not the first (8)
I believe the answer is:
infernal
'confounded' is the definition.
(thesaurus)
'plant closing borders not the first' is the wordplay.
I cannot quite see how this works, but
'plant' could be 'fern' (fern is a kind of plant) and 'fern' is found in the answer.
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is found in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for infernal that I've seen before include "To do with hell or informally tiresome" , "To do with hell, or informally detestable" , "Irritating and tiresome or relating to hell" , "Damned, diabolical" , "Feral inn - it's hellish" .)