Ornamental garden plant — holding one, state politicians squeal (6,5)
I believe the answer is:
pampas grass
'ornamental garden plant' is the definition.
(pampas grass is used as a garden plant)
'holding one state politicians squeal' is the wordplay.
'holding' is an insertion indicator.
'one' becomes 'a' (a thing is one thing).
'state' becomes 'pa' (abbreviation for Pennsylvania).
'politicians' becomes 'MPs' (Members of Parliament).
'squeal' becomes 'grass' (both can mean to act as an informant).
'pa'+'mps'='pamps'
'a' inserted inside 'pamps' is 'pampas'.
'pampas'+'grass'='PAMPAS-GRASS'
(Another definition for pampas grass that I've seen is " Tall plumed South American plant".)