Why, for the most part, entertaining fellas regularly fail to pay (5)
I believe the answer is:
welsh
'fail to pay' is the definition.
I can't tell whether this defines the answer.
'why for the most part entertaining fellas regularly' is the wordplay.
'for the most part' means to remove the last letter (most but not all of the letters).
'entertaining' indicates putting letters inside (inviting in, as for dinner).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'fellas' are 'els'.
'why' with its last letter removed is 'wh'.
'wh' going around 'els' is 'WELSH'.
Can you help me to learn more?
(Other definitions for welsh that I've seen before include "Cambrian" , "Nationality of Dai or Ivor" , "Leave without paying" , "Default on bet in Wales?" , "From a principality" .)