Strange bottom, bottom not to be seen! (3)
I believe the answer is:
rum
'strange' is the definition.
(similar in meaning)
'bottom bottom not to be seen' is the wordplay.
'bottom' becomes 'rump' (rump can informally mean someone's bottom).
'bottom not to be seen' means to remove the last letter.
'rump' with its last letter taken off is 'RUM'.
(Other definitions for rum that I've seen before include "Peculiar like sailor's drink (3)" , "Pirate's grog" , "Curious, strange" , "An alcoholic spirit - sounds odd" , "Odd kind of drink for sailor" .)