Passing tennis shot, following return of skill (9)
'passing' is the definition.
('transient' can be a synonym of 'passing')
'tennis shot following return of skill' is the wordplay.
'shot' is an anagram indicator.
'following' says to put letters next to each other.
'return of' says the letters should be written backwards.
'skill' becomes 'art' (I've seen this before).
'art' reversed gives 'tra'.
'tennis' anagrammed gives 'nsient'.
'nsient' put after 'tra' is 'TRANSIENT'.
(Other definitions for transient that I've seen before include "Fleeting", "fugitive", "Lasting little time", "Lasting a short time", "Ephemeral".)