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 definition 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' means one lot of letters goes inside another (inviting in, as for dinner).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternate letters of 'fellas' are 'els'.
'why' with its final letter removed is 'wh'.
'wh' enclosing '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" .)