It is mainly uninhabited (6,6)
I believe the answer is:
desert island
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'is mainly uninhabited' is the wordplay.
I cannot quite understand how this works, but
'is' is present in the answer.
'uninhabited' could be 'desert' (similar in meaning) and 'desert' is found in the answer.
The remaining letters 'land' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for desert island that I've seen before include "remote spot" , "Traditional scene of castaways" , "Remote tropical place - lets adders in (anag)" , "traditional place for CA [CHILDRENS ADVENTURE] - with 6d" , "____ ____ Discs, a radio programme" .)