Closed trailers bring hoppers (5)
I believe the answer is:
toads
'hoppers' is the definition.
(animals which hop)
'closed trailers' is the wordplay.
'closed' becomes 'to' (as in a door closed to).
'trailers' becomes 'ads' (a trailer is a kind of ad or advert).
'to'+'ads'='TOADS'
'bring' acts as a link.
(Other definitions for toads that I've seen before include "Froglike amphibians" , "Frog-like land animals" , "Loathsome types" , "Tailless amphibians" , "Leaping amphibians" .)