Mercurial (7)
'mercurial' is the definition.
(I know that mercurial can be written as erratic)
This is all the clue.
(Other definitions for erratic that I've seen before include "Hit-or-miss" , "Volatile" , "Uneven in movement or quality" , "Wild" , "Rambling" .)
'mercurial' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for volatile that I've seen before include "Fickle; evaporating easily" , "Liable to unpredictable change" , "Subject to sudden changes" , "Explosive or excitable" , "Unstable and liable to change" .)