Cloud's position hiding rook (7)
I believe the answer is:
stratus
'cloud's' is the definition.
(stratus is a kind of cloud)
'position hiding rook' is the wordplay.
'position' becomes 'status' (one's status or standing).
'hiding' means one lot of letters goes inside another.
'rook' becomes 'R' (chess abbreviation).
'status' going around 'r' is 'STRATUS'.
(Other definitions for stratus that I've seen before include "Cloud as a grey sheet" , "Cloud cover" , "Grey cover" , "Continuous cloud type" , "Sheet of low cloud" .)