Oddly, loud child is coherent (5)
I believe the answer is:
lucid
'coherent' is the definition.
(I've seen this before)
'oddly loud child' is the wordplay.
'oddly' indicates alternate letters.
'loud'+'child'='loudchild'
The alternating letters of 'loudchild' are 'LUCID'.
'is' is the link.
(Other definitions for lucid that I've seen before include "Shining, clearly" , "Coherent" , "Clear, easy to understand" , "Clearly expressed and altogether rational" , "Crystal clear" .)