Mineral popular in Oman’s capital, as it happens (7)
I believe the answer is:
olivine
'mineral' is the definition.
(type of mineral)
'popular in oman's capital as it happens' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'in' indicates putting letters inside.
'capital' indicates taking the first letters.
'as it happens' becomes 'live' (as in live television).
The first letter of 'omans' is 'o'.
'o'+'live'='olive'
'in' going inside 'olive' is 'OLIVINE'.
(Other definitions for olivine that I've seen before include "stone" , "Clear green gemstone" , "mineral in rock" , "Dark green mineral" .)