The most common and most likely answer to this crossword clue is the 7 plus 6 letter word COMPLEX NUMBER
I've seen this crossword clue in The Times
C
O
M
P
L
E
X
N
U
M
B
E
R
Buddy explains!
The straight part
We know that "It’s partly imaginary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s partly imaginary = complex number.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.