Crossword Genius

Pulls small locomotives (7)

Ross

I believe the answer is:

strains

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'pulls' is the definition.
The answer and definition can be both acts as well as being plural nouns.
Maybe there's a link between them I don't understand?

'small locomotives' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'locomotives' becomes 'trains'.
's'+'trains'='STRAINS'

Can you help me to learn more?

(Other definitions for strains that I've seen before include "Tries very hard" , "Melodies" , "Stresses and - -" .)

I've seen this clue in The Times.
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