Female detective in time becomes mediocre (7)
I believe the answer is:
average
'mediocre' is the definition.
(I know that mediocre can be written as average)
'female detective in time' is the wordplay.
'female detective in' becomes 'aver' (I can't justify this - if you can you should believe this answer much more).
'time' becomes 'age' (an age is a period of time).
'aver'+'age'='AVERAGE'
'becomes' is the link.
Can you help me to learn more?
(Other definitions for average that I've seen before include "Normal in arithmetic way" , "Mediocre, commonplace" , "Arithmetically ordinary?" , "Of no exceptional quality or ability" , "About normal or usual" .)