The most common and most likely answer to this crossword clue is the 11 letter word ROMANTICISE
I've seen this crossword clue in The Daily Telegraph
R
O
M
A
N
T
I
C
I
S
E
Buddy explains!

The straight part
We know that "Paint a fanciful picture" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case paint a fanciful picture = romanticise.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.
Crossword Answer definitions
ROMANTICISE verb- To interpret or view something in a romantic (unrealistic, idealized) manner.
- To think or act in a romantic manner.