Declare silver base is mediocre (7)
I believe the answer is:
average
'mediocre' is the definition.
(I know that mediocre can be written as average)
'declare silver base' is the wordplay.
'declare' becomes 'aver' (averring is a kind of declaring).
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'base' becomes 'e' (Euler's number - base of natural logarithms).
'aver'+'ag'+'e'='AVERAGE'
'is' acts as a link.
(Other definitions for average that I've seen before include "Mean, so-so" , "Normal in arithmetic way" , "Arithmetical mean" , "Fair to middling" , "Mean - everyday" .)