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I tried

    In[1]:= Subscript[a, 0] = 1
    Out[1]= 1

    In[2]:= Clear[Subscript[a, 0]]
    During evaluation of In[452]:= Clear::ssym: Subscript[a, 0] is not a symbol or a string. >>

    In[3]:= Clear[a]
 
    In[4]:= Subscript[a, 0]
    Out[4]= 1

Any idea?

I tried

Subscript[a, 0] = 1
(* 1 *)

Clear[Subscript[a, 0]]

       During evaluation of Clear::ssym: Subscript[a, 0] is not a symbol or a string. >>

Clear[a]
Subscript[a, 0]
(* 1 *)

Any idea?