Stuffy royal seen in road with a smaller amount (7)
I believe the answer is:
airless
'stuffy' is the definition.
(stuffy means lacking ventilation)
'royal seen in road with a smaller amount' is the wordplay.
'royal' becomes 'r' (abbreviation).
'seen in' means one lot of letters goes inside another.
'road' becomes 'AI' (resembles A1, a major UK road).
'with' means one lot of letters go next to another.
'a smaller amount' becomes 'less' (I've seen this in another clue).
'ai'+'less'='ailess'
'r' put within 'ailess' is 'AIRLESS'.
(Other definitions for airless that I've seen before include "Stifling" , "Close" , "Still" , "Lacking ventilation" , "not a breath of wind" .)