In bed, a pretty little thing (4)
I believe the answer is:
bead
'pretty little thing' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'in bed a' is the wordplay.
'in' means one lot of letters goes inside another.
'bed' enclosing 'a' is 'BEAD'.
Can you help me to learn more?
(Other definitions for bead that I've seen before include "bit off necklace, say" , "Small drop of moisture" , "Round piece of glass, etc" , "Drop, part of rosary" , "Small ball with a hole through the middle" .)