Diver on Channel Islands recalled shipping hazard (7)
I believe the answer is:
iceberg
'shipping hazard' is the definition.
(I've seen this before)
'diver on channel islands recalled' is the wordplay.
'diver' becomes 'grebe' (diving bird).
'on' says to put letters next to each other.
'channel islands' becomes 'ci'.
'recalled' shows that the letters should be reversed in order.
'grebe'+'ci'='grebeci'
'grebeci' written backwards gives 'ICEBERG'.
(Other definitions for iceberg that I've seen before include "Large floating mass of frozen water" , "Titanic breaker" , "Calf's parent" , "Large mass of floating ice" , "Mass carved from a glacier" .)