Lettuce is a shipping hazard (7)
I believe the answer is:
iceberg
I believe this clue is a double definition.
'lettuce' is the first definition.
(iceberg lettuce is a kind of lettuce)
'a shipping hazard' is the second definition.
(I've seen this before)
'is' is the link.
(Other definitions for iceberg that I've seen before include "arctic feature" , "Maritime danger, mostly underwater" , "One destroyed the Titanic" , "Frozen floater" , "A floating ice mountain" .)