People going under — not completely different (6)
I believe the answer is:
divers
'people going under' is the definition.
(divers go under the sea)
'not completely different' is the wordplay.
'not completely' means to remove the last letter (the word is incomplete).
'different' becomes 'diverse' (similar in meaning).
'diverse' with its last letter taken away is 'DIVERS'.
(Other definitions for divers that I've seen before include "footballers cheating" , "ducks" , "Some water-birds; assorted" , "Various for underwater workers?" , "Many and different" .)