Firstly I reside. Secondly the letter exists towards the expression resolution.
Notes: The above two sentences fully and unambiguously describe the answer. Also, I'm not sure if this qualifies as a riddle.
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Each letter of the words spells out
F resides first in the sentence, and secondly it is a part of the resolution of the answer, being in FIRSTLETTER.
The answer is
which is cool because
the title suggests that the answer is the key, and that is true - the answer tells you how to solve the question (by taking the first letter of every word)
Firstly I reside
This isn't a cryptic clue, it tells you where to look for the answer, as in I reside first. Also a subtle hint that the answer begins with first.
Secondly the letter exists towards the expression resolution.
Secondly the letter exists towards the expression resolution, which is also true because letter is the second word in the phrase.
To expand on other answers:
"Firstly I reside": This could refer to the letter I, which looks like the upright part of the letter F.
"Secondly the letter exists towards the expression resolution.": This could refer to an equals sign in a mathematical expression and describ the horizontal bars of the F.