Been lying, on the quiet, as is obvious (5)
I believe the answer is:
plain
'is obvious' is the definition.
(similar in meaning)
'been lying on the quiet' is the wordplay.
I cannot quite see how this works, but
'been' could be 'lain' (lying is a kind of being) and 'lain' is located in the answer.
'quiet' could be 'p' (music symbol for 'piano', ie soft or quiet) and 'p' is found within the answer.
No letters remain.
This explanation may well be incorrect...
'as' is the link.
Can you help me to learn more?
(Other definitions for plain that I've seen before include "Geographical feature" , "Basic" , "Undecorated; clear" , "lament" , "lacking special beauty" .)