Attempt to retain tree that's worthless (6)
I believe the answer is:
trashy
'worthless' is the definition.
(trashy can mean of poor quality)
'attempt to retain tree' is the wordplay.
'attempt to' becomes 'try' (I've seen this in other clues).
'retain' is an insertion indicator.
'tree' becomes 'ash tree' (ash is a kind of tree).
'try' enclosing 'ash' is 'TRASHY'.
'that's' acts as a link.
(Other definitions for trashy that I've seen before include "Cheap, shoddy" , "Shoddy and worthless" , "As rubbish" , "Of no value or quality" , "Shy art - rubbishy" .)