Cryptic crossword clue
Sons watch a locomotive 7

The most common and most likely answer to this crossword clue is the 7 letter word SHUNTER

I've seen this crossword clue in Independent
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The straight part
We know that "a locomotive" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a locomotive = shunter.
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 SHUNTER mean?

SHUNTER noun
  1. A railway locomotive used for shunting; a switcher.
  2. A person who carries out shunting operations.

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