Full Idea
Davidson thinks that Frege's model for a theory of semantic value (and thereby for a systematic theory of sense) is unsatisfactory, because it provides no useful or explanatory account of how sentence-meaning can be a function of word-meaning.
Gist of Idea
Davidson thinks Frege lacks an account of how words create sentence-meaning
Source
report of Donald Davidson (Truth and Meaning [1967]) by Alexander Miller - Philosophy of Language 8.1
Book Reference
Miller,Alexander: 'Philosophy of Language' [UCL Press 1998], p.246
A Reaction
Put like that, it is not clear to me how you could even start to explain how word-meaning contributes to sentence meaning. Try speaking any sentence slowly, and observe how the sentence meaning builds up. Truth is, of course, relevant.