Garden feature mostly separated — blunder admitted (8)
I believe the answer is:
parterre
'garden feature' is the definition.
'parterre' can be an answer for 'garden' (I've seen this before). I am unsure of the 'feature' bit.
'mostly separated blunder admitted' is the wordplay.
'mostly separated' becomes 'parte' (I can't explain this - if you can you should give a lot more credence to this answer).
'blunder' becomes 'err' (to err is to make a mistake or blunder).
'admitted' is an insertion indicator (inserted letters are let in or admitted).
'parte' enclosing 'err' is 'PARTERRE'.
Can you help me to learn more?
(Other definitions for parterre that I've seen before include "Formally patterned flower garden" , "level ground" , "Formal garden feature" , "Ornamental arrangement of flower beds and paths -- rarer pet (anag)" , "Theatre seating area on the ground floor" .)