A child's first coin, in a manner of speaking (6)
I believe the answer is:
accent
'a manner of speaking' is the definition.
(accent is a kind of manner of speaking)
'a child's first coin in' is the wordplay.
'child's first' becomes 'c' (1st letter of 'child').
'coin' becomes 'cent' (cent is a kind of coin).
'in' is an insertion indicator.
'c' going within 'cent' is 'ccent'.
'a'+'ccent'='ACCENT'
(Other definitions for accent that I've seen before include "Place stress on, say a syllable" , "pitch" , "Grave, perhaps" , "Emphasis on syllable or local pronunciation" , "Distinctive way of pronouncing language" .)