Not many at sea? It means the opposite (7)
I believe the answer is:
antonym
'it means the opposite' is the definition.
(I've seen this before)
'not many at sea?' is the wordplay.
'at sea?' is an anagram indicator (I've seen 'at sea' mean this).
'not'+'many'='notmany'
'notmany' is an anagram of 'ANTONYM'.
(Other definitions for antonym that I've seen before include "A word opposite in meaning to another" , "For villain, hero is one" , ""Loud" as opposed to "quiet", for example" , "One's import is contrary" , "opposing term" .)