The most common and most likely answer to this crossword clue is the 5 letter word CHUNK
I've seen this crossword clue in The Daily Telegraph
C
H
U
N
K
Buddy explains!
The straight part
We know that "large portion" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case large portion = chunk.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 CHUNK mean?
CHUNK noun- A part of something that has been separated.
- A representative portion of a substance, often large and irregular.
- A sequence of two or more words that occur in language with high frequency but are not idiomatic; a bundle or cluster.
- A discrete segment of a file, stream, etc. (especially one that represents audiovisual media); a block.
- A segment of a comedian's performance
- To break into large pieces or chunks.
- To break down (language, etc.) into conceptual pieces of manageable size.
- To throw.