TV sleuth in time becomes mediocre (7)
I believe the answer is:
average
'mediocre' is the definition.
(I know that mediocre can be written as average)
'tv sleuth in time' is the wordplay.
'tv sleuth' becomes 'vera' (I can't justify this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'time' becomes 'age' (an age is a period of time).
'vera' placed inside 'age' is 'AVERAGE'.
'becomes' is the link.
Can you help me to learn more?
(Other definitions for average that I've seen before include "It may be mean" , "Arithmetically ordinary?" , "Mean - typical" , "just like any other" , "Of no exceptional quality or ability" .)