The most common and most likely answer to this crossword clue is the 11 letter word HALLUCINATE
I've seen this crossword clue in The Times
H
A
L
L
U
C
I
N
A
T
E
Buddy explains!

The straight part
We know that "See things" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case see things = hallucinate.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 HALLUCINATE mean?
HALLUCINATE verb- To seem to perceive things (with one or more of one's senses) which are not really present; to have visions; to experience a hallucination.