The most common and most likely answer to this crossword clue is the 9 letter word COGNITIVE
I've seen this crossword clue in Financial Times
C
O
G
N
I
T
I
V
E
Buddy explains!
The straight part
We know that "one encountered during Viet Cong manoeuvres (9)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one encountered during viet cong manoeuvres (9) = cognitive.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 COGNITIVE mean?
COGNITIVE noun- Cognate.
- Relating to the part of mental functions that deals with logic, as opposed to affective which deals with emotions.
- Intellectual.
- Cognate; which is to be recognized as cognate.