Becoming a medium, you say, takes time (7)
I believe the answer is:
average
'becoming a medium you' is the definition.
I can't judge whether this definition defines the answer.
'say takes time' is the wordplay.
'say' becomes 'aver' ('aver' can be a synonym of 'say').
'takes' says to put letters next to each other.
'time' becomes 'age' (age is a kind of time).
'aver'+'age'='AVERAGE'
Can you help me to learn more?
(Other definitions for average that I've seen before include "Medial" , "About normal or usual" , "Normal amount" , "Mean - typical" , "representative figure?" .)