He’ll help with an idea (4)
I believe the answer is:
aide
'he'll' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'help with an idea' is the wordplay.
'help' becomes 'aid' ('aid' can be a synonym of 'help').
'with' says to put letters next to each other.
'an idea' becomes 'e' (I am not sure about this - if you are sure you should believe this answer much more).
'aid'+'e'='AIDE'
Can you help me to learn more?
(Other definitions for aide that I've seen before include "Leader's assistant" , "Officer's assistant" , "Second" , "Assistant to a more senior person" , "Right-hand man/woman" .)