Sense hawk is circling top of mountain (5)
I believe the answer is:
smell
'sense' is the definition.
(I know that sense can be written as smell)
'hawk is circling top of mountain' is the wordplay.
'hawk' becomes 'sell' (hawk can mean to sell or trade).
'is circling' indicates putting letters inside (circling can mean going around or containing).
'top of' indicates taking the first letters.
The first letter of 'mountain' is 'm'.
'sell' enclosing 'm' is 'SMELL'.
(Other definitions for smell that I've seen before include "Nasal sense" , "stink" , "Hum" , "Aroma" , "Scent" .)