The most common and most likely answer to this crossword clue is the 6 letter word MUSING
M
U
S
I
N
G
Buddy explains!
The straight part
We know that "Thinking" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thinking = musing.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 MUSING mean?
MUSING verb- To become lost in thought, to ponder.
- To say (something) with due consideration or thought.
- To think on; to meditate on.
- To wonder at.
- Thought, meditation, contemplation
- Absorbed in thought; contemplative