Cryptic crossword clue
They act unthinkingly, placing plant needing warmth in a sickly sun 10

The most common and most likely answer to this crossword clue is the 10 letter word AUTOMATONS

I've seen this crossword clue in The Daily Telegraph
A
U
T
O
M
A
T
O
N
S

Buddy explains!

The straight part
We know that "They act unthinkingly," is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they act unthinkingly, = automatons.
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 AUTOMATONS mean?

AUTOMATONS noun
  1. A machine or robot designed to follow a precise sequence of instructions.
  2. A person who acts like a machine or robot, often defined as having a monotonous lifestyle and lacking in emotion.
  3. A formal system, such as finite automaton.
  4. A toy in the form of a mechanical figure.
  5. The self-acting power of the muscular and nervous systems, by which movement is effected without intelligent determination.

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