Full Idea
It is a mistake to regard scheme (T) as a definition of truth.
Clarification
Scheme (T) is 'X is true iff p'
Gist of Idea
Scheme (T) is not a definition of truth
Source
Alfred Tarski (The Semantic Conception of Truth [1944], 15)
Book Reference
'Semantics and the Philosophy of Language', ed/tr. Linsky,Leonard [University of Illinois 1972], p.29
A Reaction
The point is, I take it, that the definition is the multitude of sentences which are generated by the schema, not the schema itself.