Full Idea
In any first-order language, there are infinitely many T-sentences. Since definitions should be finite, the agglomeration of all the T-sentences is not a definition of truth.
Clarification
A T-sentence of the form: 'p' is true iff p
Gist of Idea
A first-order language has an infinity of T-sentences, which cannot add up to a definition of truth
Source
William D. Hart (The Evolution of Logic [2010], 4)
Book Reference
Hart,W.D.: 'The Evolution of Logic' [CUP 2010], p.101
A Reaction
This may be a warning shot aimed at Davidson's extensive use of Tarski's formal account in his own views on meaning in natural language.