Are not in use, seemingly (10)
I believe the answer is:
apparently
'seemingly' is the definition.
('apparently' can be a synonym of 'seemingly')
'are not in use' is the wordplay.
'are' becomes 'a' (the 'are' is a metric unit of area).
'not' becomes 'parent' (I can't justify this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'use' becomes 'ply' (plying is a kind of using).
'parent' put inside 'ply' is 'pparently'.
'a'+'pparently'='APPARENTLY'
Can you help me to learn more?
(Other definitions for apparently that I've seen before include "On the face of it" , "Seemingly" , "Clearly" , "so it seems" .)