Old boss to stock new shrub (8)
I believe the answer is:
oleander
'shrub' is the definition.
(I've seen this before)
'old boss to stock new' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'boss' becomes 'leader' (I've seen this before).
'to stock' is an insertion indicator (some letters contain or stock others).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'leader' enclosing 'n' is 'leander'.
'o'+'leander'='OLEANDER'
(Other definitions for oleander that I've seen before include "No leader, oddly, for the flowering shrub" , "Flowering plant" , "Adore Len (anag.)" , "Red alone (anag.)" , "Poisonous shrub, also called rosebay" .)