Lying in one (4)
I believe the answer is:
abed
'lying' is the definition.
(lying abed or in bed)
'one' is the wordplay.
I cannot quite see how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is located in the answer.
The remaining letters 'bed' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'in' acts as a link.
Can you help me to learn more?
(Other definitions for abed that I've seen before include "Bead (anag.)" , "Yet to rise" , "Having not got up" , "going to sleep?" , "Sleeping under covers" .)