Largest one in the middle is wet (5)
I believe the answer is:
moist
'is wet' is the definition.
(similar in meaning)
'largest one in the middle' is the wordplay.
'largest' becomes 'most' (I can't justify this - if you can you should believe this answer much more).
'one' becomes 'i' (Roman numeral).
'in the middle' is an insertion indicator.
'most' enclosing 'i' is 'MOIST'.
Can you help me to learn more?
(Other definitions for moist that I've seen before include "Slightly wet, damp" , "bit wet" , "Damp, humid" , "Dampish" , "Affected by tears, perhaps" .)