The girl and boy had a certain rhythm (6)

I believe the answer is:

lilted

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'had a certain rhythm' is the definition.
The definition and answer can be both to do with communicating as well as being past participle verbs.
Perhaps they are linked in a way I don't understand?

'the girl and boy' is the wordplay.
'the girl' becomes 'lil' ().
'and' is a charade indicator (letters next to each other).
'boy' becomes 'Ted' (short for Edward or Theodore).
'lil'+'ted'='LILTED'

Can you help me to learn more?

(Another definition for lilted that I've seen is " humming".)

I've seen this clue in the Evening Standard.

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