Lettuce that may be a danger to shipping (7)
I believe the answer is:
iceberg
I believe this is a double definition.
'lettuce' is the first definition.
(I know that iceberg is a type of lettuce)
'a danger to shipping' is the second definition.
(I've seen this before)
'that may be' acts as a link.
(Other definitions for iceberg that I've seen before include "danger at sea" , "Type of floe" , "sort of lettuce" , "Unlikely feature of Cos" , "calf, perhaps" .)