Keep quiet and polish plant (5)
I believe the answer is:
shrub
'plant' is the definition.
(type of plant)
'keep quiet and polish' is the wordplay.
'keep quiet' becomes 'sh' (as in shushing someone).
'and' means one lot of letters go next to another.
'polish' becomes 'rub' (polish meaning rub to make shiny).
'sh'+'rub'='SHRUB'
(Other definitions for shrub that I've seen before include "Woody plant smaller than a tree" , "Hazel perhaps" , "Perhaps Heather" , "Plant - brush (anag)" , "Cordial" .)