The most common and most likely answer to this crossword clue is the 2 plus 5 letter word ON PAPER
I've seen this crossword clue in The Daily Telegraph
O
N
P
A
P
E
R
More clues leading to the result ON PAPER
- Person right to consume trivial entertainment, in theory
- In theory, paid for by The Times?
- Theoretically working daily?
- Propane explodes, theoretically
- As printed, this could make propane
- Figure eating puree, with recipe written down
- It could be written down in theory
- In theory, propane explodes
- Where one might find diarist in theory
- It’s to do with exam in theory
Buddy explains!

The straight part
We know that "in theory" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in theory = on paper.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.