Naval officers raised alarm at sea after first sign of rocks (4,8)
I believe the answer is:
'naval officers' is the definition.
'raised alarm at sea after first sign of rocks' is the wordplay.
'at sea' indicates an anagram.
'after' means one lot of letters go next to another.
'first sign of' says to take the initial letters (I've seen 'first signs' mean this).
The first letter of 'rocks' is 'r'.
'raisedalarm' with letters rearranged gives 'earadmirals'.
'earadmirals' after 'r' is 'REAR-ADMIRALS'.