The most common and most likely answer to this crossword clue is the 11 letter word HALF-LENGTH
I've seen this crossword clue in Financial Times
H
A
L
F
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L
E
N
G
T
H
More clues leading to the result HALF-LENGTH
Buddy explains!
The straight part
We know that "Portrait of upper body" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case portrait of upper body = half-length.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 HALF-LENGTH mean?
HALF-LENGTH noun- Half the length of a horse.
- A portrait or photograph that depicts only the upper half of the subject.
- Half the normal length.
- Of a portrait or photograph: depicting only the upper half of the subject.