Dull hosts run trading centre (4)
I believe the answer is:
mart
'trading centre' is the definition.
(I've seen this before)
'dull hosts run' is the wordplay.
'dull' becomes 'mat' (mat as opposed to glossy).
'hosts' means one lot of letters goes inside another.
'run' becomes 'r' (cricket abbreviation).
'mat' going around 'r' is 'MART'.
(Other definitions for mart that I've seen before include "Place" , "Trading centre" , "US shop; market" , "Take tram to market" .)