Obviously a unit of area (3)
I believe the answer is:
are
'unit of area' is the definition.
'are' can be an answer for 'unit' (are is a kind of unit). I'm not certain of the 'of area' bit.
'obviously a' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
The remaining letters 're' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for are that I've seen before include "live" , "is for more than one" , "Exist; square measure" , "(They/you/we) exist" , "Era (anag.)" .)