Crossword Genius

Business people strong on safety first (5)

Ross

I believe the answer is:

firms

I'm a little stuck... Click here to teach me more about this clue!

'business people' is the definition.
Although both the answer and definition are plural nouns, I cannot understand how one could define the other.

'strong on safety first' is the wordplay.
'strong' becomes 'firm' (synonyms).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'safety first' becomes 's' (1st letter of 'safety').
'firm'+'s'='FIRMS'

Can you help me to learn more?

(Another definition for firms that I've seen is " Companies".)

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