Damp haze enveloping capital in Oman (5)
I believe the answer is:
moist
'damp' is the definition.
(synonyms)
'haze enveloping capital in oman' is the wordplay.
'haze' becomes 'mist' (I've seen this before).
'enveloping' indicates putting letters inside.
'capital in' says to take the initial letters.
The first letter of 'oman' is 'o'.
'mist' placed around 'o' is 'MOIST'.
(Other definitions for moist that I've seen before include "Damp, humid" , "sloppy" , "Damp or clammy" , "Slightly wet, damp" , "bit wet" .)