Guardian follows one into unlit derelict submarine (8)
I believe the answer is:
nautilus
'submarine' is the definition.
(submarine in 20,000 Leagues Under the Sea)
'guardian follows one into unlit derelict' is the wordplay.
'guardian' becomes 'us' (the crossword setter and their Guardian colleagues).
'follows' is a charade indicator (letters next to each other).
'one' becomes 'a' (a thing is one thing).
'into' indicates putting letters inside.
'derelict' is an anagram indicator (a derelict or broken spelling).
'unlit' anagrammed gives 'nutil'.
'a' going inside 'nutil' is 'nautil'.
'us' after 'nautil' is 'NAUTILUS'.
(Other definitions for nautilus that I've seen before include "'Pearly' mollusc" , "Cephalopod mollusc" , "Creature of deep" , "Usual tin for a small sea creature" , "Mollusc and first nuclear-powered submarine" .)