The most common and most likely answer to this crossword clue is the 6 letter word INTENT
I've seen this crossword clue in The Guardian
I
N
T
E
N
T
More clues leading to the result INTENT
Buddy explains!

The straight part
We know that "Firm/plan" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case firm/plan = intent.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does INTENT mean?
INTENT noun- A purpose; something that is intended.
- The state of someone’s mind at the time of committing an offence.
- Firmly fixed or concentrated on something.
- Engrossed.
- Unwavering from a course of action.