Full Idea
In his middle period, Frege became hostile to contextual definitions, and any definition other than an explicit one, ..but at the time of the 'Grundlagen' he conceived of his context principle as licensing contextual definitions.
Clarification
His middle period is 1891-1906
Gist of Idea
Originally Frege liked contextual definitions, but later preferred them fully explicit
Source
report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Michael Dummett - Frege philosophy of mathematics Ch.11
Book Reference
Dummett,Michael: 'Frege: philosophy of mathematics' [Duckworth 1991], p.125
A Reaction
His context principle says words only have a meaning in a context. Intuitively, I would say that there is no correct answer to how something should be defined. Totally circularity is hopeless, but presuppositions just weaken a definition.