Sitting in position around study, heading off change (9)
I believe the answer is:
sedentary
'sitting' is the definition.
(thesaurus)
'in position around study heading off change' is the wordplay.
'in position' becomes 'set' (I've seen this in another clue).
'around' indicates putting letters inside.
'study' becomes 'den' (term for a study or office).
'heading off' means to remove the first letter.
'change' becomes 'vary' (synonyms).
'set' placed around 'den' is 'sedent'.
'vary' with its first letter taken off is 'ary'.
'sedent'+'ary'='SEDENTARY'
(Other definitions for sedentary that I've seen before include "Sitting a lot and taking little exercise" , "inactive" , "Tending to spend much time seated" , "Sitting much and taking little exercise" , "Tending to spend too much seated and taking little exercise" .)