This Sea is also a lake (like the Caspian) (4)
I believe the answer is:
aral
'this sea' is the definition.
(I've seen this before)
'also a lake like the caspian' is the wordplay.
I cannot quite see how this works, but
'a' is present in the answer.
'lake' could be 'l' (geographical abbreviation) and 'l' is present in the answer.
The remaining letters 'ra' 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).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for aral that I've seen before include "Large lake of Central Asia" , "lnland sea, fourth largest lake in the world" .)