Vehicles essentially picking up diver in danger at sea (7)
I believe the answer is:
iceberg
'danger at sea' is the definition.
(I've seen this before)
'vehicles essentially picking up diver' is the wordplay.
'essentially' indicates the central letters.
'picking up' says the letters should be written in reverse (in a down clue, letters go up).
'diver' becomes 'grebe' (diving bird).
The middle of 'vehicles' is 'ic'.
'grebe' in reverse letter order is 'eberg'.
'ic'+'eberg'='ICEBERG'
'in' acts as a link.
(Other definitions for iceberg that I've seen before include "Huge mass of frozen water" , "Mass carved from a glacier" , "Polar shipping hazard" , "Largely hidden danger" , "shunned by sailors?" .)