Happen, as a plural form, to include the singular (5)
I believe the answer is:
arise
'happen' is the definition.
(arising is a kind of happening)
'a plural form to include the singular' is the wordplay.
'a plural form' becomes 'arie' (I can't justify this - if you can you should give a lot more credence to this answer).
'to include' indicates putting letters inside.
'the singular' becomes 's' (abbreviation sometimes used in dictionaries).
'arie' going around 's' is 'ARISE'.
'as' is the link.
Can you help me to learn more?
(Other definitions for arise that I've seen before include "Crop up" , "Tower" , "Occur; stand up" , "Wake up or get up" , "W B Yeats: 'I will . . . . . and go now'" .)