Bare bottoms in stream are rather large (4)
I believe the answer is:
mere
'bare' is the definition.
(I know that bare can be written as mere)
'bottoms in stream are rather large' is the wordplay.
'bottoms in' suggests the final letters (I've seen 'bottom in' mean this (in a down clue, the letter at the bottom)).
The last letters of 'stream are rather large' are 'MERE'.
(Other definitions for mere that I've seen before include "Bare, simple" , "See 13" , "Pure; lake" , "Lake, pond (poetic)" , "bathing may be permitted in this?" .)