Complete sentence child finally spoke (7)
I believe the answer is:
uttered
'spoke' is the definition.
('utter' can be a synonym of 'speak')
'complete sentence child finally' is the wordplay.
'complete' becomes 'utter' (both can mean total).
'finally' indicates one should take the final letters.
The last letters of 'sentence child' are 'ed'.
'utter'+'ed'='UTTERED'
(Other definitions for uttered that I've seen before include "Said (something)" , "Spoke and put into circulation" , "Pronounced" , "In law, put forged money into circulation" , "Spoken" .)