He has a treatment for infantile insomnia (7)
I believe the answer is:
sandman
'he has a treatment for infantile insomnia' is the definition.
The definition suggests a singular noun which matches the answer.
'he has a' is the wordplay.
'he' becomes 'sandm' (I can't justify this - if you can you should believe this answer much more).
'has' is a charade indicator (letters next to each other).
'a' becomes 'an'.
'sandm'+'an'='SANDMAN'
Can you help me to learn more?
(Other definitions for sandman that I've seen before include "He comes to send children off" , "Children's bedtime fairy" , "Imaginary person who causes children to sleep" , "One of the Beach Boys" , "Who could send you to sleep" .)