The most common and most likely answer to this crossword clue is the 13 letter word CONTEMPLATIVE
I've seen this crossword clue in The Times
C
O
N
T
E
M
P
L
A
T
I
V
E
Buddy explains!
The straight part
We know that "Thinking of" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thinking of = contemplative.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 CONTEMPLATIVE mean?
CONTEMPLATIVE noun- Someone who has dedicated themselves to religious contemplation.
- Inclined to contemplate; introspective and thoughtful; meditative.
- Pertaining to a religious contemplative, or a contemplative religious orders, especially the Roman Catholic varieties.
- Relating to, or having the power of, contemplation.