Assist artfully, catching ball close to boundary showing equilibrium (8)
I believe the answer is:
isostasy
'equilibrium' is the definition.
(I have seen 'Geological equilibrium' mean 'isostasy' so perhaps 'equilibrium' could also mean 'isostasy')
'assist artfully catching ball close to boundary' is the wordplay.
'artfully' is an anagram indicator.
'catching' indicates putting letters inside.
'ball' becomes 'o'.
'close to' says to take the final letters (the close/ending of).
The last letter of 'boundary' is 'y'.
'assist' is an anagram of 'isstas'.
'isstas' going around 'o' is 'isostas'.
'isostas'+'y'='ISOSTASY'
'showing' is the link.
(Other definitions for isostasy that I've seen before include "earthly balance" , "Geological equilibrium" .)