一定: a maintained value and specified conditions
Distinguish 一定に保つ from 一定の条件 in context. An adjustment intended to maintain a value and a conditional ability to apply do not both describe completed outcomes.
一定に保つ aims to maintain a value
一定 is read いってい. 室温を一定に保つよう暖房を調整した。 says that someone adjusted the heating to keep the room temperature constant.
The value to be maintained here is the room temperature. Adjusting the heating was completed, but the sentence does not confirm that the temperature actually stayed constant afterward.
一定の条件 names specified conditions
一定の条件を満たせば申し込みができる。 says that someone can apply if the specified conditions are met. Here, 一定 describes conditions that have been set.
満たせば introduces the condition if they are met, and できる expresses ability or possibility. The sentence alone does not establish that a particular person met them or completed an application.
Identify what 一定 describes
The temperature sentence gives a goal of maintaining a value. The application sentence gives specified conditions for applying. The word 一定 alone does not make the two subjects the same kind of item.
Check the noun associated with 一定 and the action that follows. Read the maintenance goal, fulfillment of conditions and possibility of applying within the scope supplied by each sentence.
Does 一定の条件を満たせば申し込みができる mean an application has already been completed?
Show the answer and why
No. It gives a condition and the possibility of applying if that condition is met. Actual fulfillment and application completion require separate confirmation.