Plant without root’s outside under bone (7)
I believe the answer is:
ribwort
'plant' is the definition.
(ribwort is a kind of plant)
'without root's outside under bone' is the wordplay.
'without' becomes 'wo' (abbreviation of 'without', normally with a slash).
'root's outside' becomes 'rt' (I can't justify this - if you can you should believe this answer much more).
'under' means one lot of letters go next to another.
'bone' becomes 'rib' (rib is a kind of bone).
'wo'+'rt'='wort'
'wort' after 'rib' is 'RIBWORT'.
(Other definitions for ribwort that I've seen before include "Type of plantain" , "weed" .)