The most common and most likely answer to this crossword clue is the 5 letter word LEMMA
I've seen this crossword clue in The Times
L
E
M
M
A
More clues leading to the result LEMMA
- Liberal introducing novel proposition
- Lexicographer's beginning with novel headword
- Novel’s end, novel’s theme
- Proposition from Frenchman, then married in the country
- Proposition presenting some problem, mathematically
- Proposition two males in meadow
- Proposition left to Austen heroine?
- First lover of Austen’s heroine gets preliminary proposition
- Liberal Victorian heroine makes a proposition
- Secondary proposition from man docked on lunar module
Buddy explains!

The straight part
We know that "theme" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case theme = lemma.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 LEMMA mean?
LEMMA noun- A proposition proved or accepted for immediate use in the proof of some other proposition.
- The canonical form of an inflected word; i.e., the form usually found as the headword in a dictionary, such as the nominative singular of a noun, the bare infinitive of a verb, etc.
- (psycholinguistics) The theoretical abstract conceptual form of a word, representing a specific meaning, before the creation of a specific phonological form as the sounds of a lexeme, which may find representation in a specific written form as a dictionary or lexicographic word.
- The outer shell of a fruit or similar body.
- One of the specialized bracts around the floret in grasses.