Mediocre woman appearing in time (7)
I believe the answer is:
average
'mediocre' is the definition.
('average' can be a synonym of 'mediocre')
'woman appearing in time' is the wordplay.
'woman' becomes 'vera' (a woman's name).
'appearing in' is an insertion indicator.
'time' becomes 'age' (age is a kind of time).
'vera' going into 'age' is 'AVERAGE'.
(Other definitions for average that I've seen before include "It may be mean" , "Middle value, the norm" , "par" , "Mean - everyday" , "Fair to middling" .)