It warns sailors after goof swims across harbour (7)
I believe the answer is:
foghorn
'it warns' is the definition.
The definition suggests a singular noun which matches the answer.
'sailors after goof swims across harbour' is the wordplay.
'sailors' becomes 'RN' (abbreviation for Royal Navy).
'after' is a charade indicator (letters next to each other).
'swims' is an anagram indicator.
'across' is an insertion indicator.
'harbour' becomes 'h' (this might be a standard abbreviation which I don't know about).
'goof' is an anagram of 'fogo'.
'fogo' going around 'h' is 'fogho'.
'rn' after 'fogho' is 'FOGHORN'.
Can you help me to learn more?
(Other definitions for foghorn that I've seen before include "Warning for ships" , "Ship warner" , "Warning device" , "Ship's hooter" , "Loud, deep, resounding voice" .)