Sudden drop in trade, nothing being imported (5)
I believe the answer is:
swoop
'sudden drop' is the definition.
(as in swooping down)
'trade nothing being imported' is the wordplay.
'trade' becomes 'swop' (synonyms).
'nothing' becomes 'o' (looks like zero - 0).
'being imported' means one lot of letters goes inside another.
'swop' placed around 'o' is 'SWOOP'.
'in' acts as a link.
(Other definitions for swoop that I've seen before include "Rush suddenly" , "Police raid" , "Descend suddenly (on)" , "Move rapidly down" , "Dive downwards" .)