Shut up and polish the bush (5)
I believe the answer is:
shrub
'bush' is the definition.
(I know that bush can be written as shrub)
'shut up and polish' is the wordplay.
'shut up' becomes 'sh'.
'and' is a charade indicator (letters next to each other).
'polish' becomes 'rub' (polish meaning rub to make shiny).
'sh'+'rub'='SHRUB'
'the' is the link.
(Other definitions for shrub that I've seen before include "Brush? (anagr.)" , "Bushy plant" , "Perhaps Heather" , "box, say" , "Mixed drink of rum, sugar, fruit-juice and spice" .)