Watery droplets filled with oxygen (5)
I believe the answer is:
moist
'watery' is the definition.
'droplets filled with oxygen' is the wordplay.
'droplets' becomes 'mist' (droplets of water vapour).
'filled with' means one lot of letters goes inside another.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'mist' going around 'o' is 'MOIST'.
(Other definitions for moist that I've seen before include "Affected by tears, perhaps" , "bit wet" , "lubricated" , "Dampish" , "Damp, humid" .)