The majority, getting out of line, are a bit wet (5)
I believe the answer is:
moist
'a bit wet' is the definition.
(I've seen this before)
'the majority getting out of line' is the wordplay.
'the majority' becomes 'most' (most of a group).
'getting' is an insertion indicator.
'out of line' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'most' going around 'i' is 'MOIST'.
'are' acts as a link.
Can you help me to learn more?
(Other definitions for moist that I've seen before include "lubricated" , "sloppy" , "bit wet" , "Slightly wet, damp" , "Damp, humid" .)