Crossword Genius

Entry ticket to the health resorts making the first son move south (4)

Ross

I believe the answer is:

pass

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

'entry ticket' is the definition.
(I've seen this in another clue)

'the health resorts making the first son move south' is the wordplay.
I cannot quite understand how this works, but
'first' could be 'a' and 'a' is found within the answer.
'son' could be 'ss' (I've seen this in other clues) and 'ss' is located in the answer.
A single letter 'p' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...

'to' is the link.

Can you help me to learn more?

(Other definitions for pass that I've seen before include "Authorise; overtake" , "Spend (19, say) -- don't know" , "Go by; permit" , "entry document" , "Exam success" .)

I've seen this clue in The Hindu.
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