Coming in holding that isn’t regularly amusing (12)
I believe the answer is:
entertaining
'amusing' is the definition.
(I've seen this before)
'coming in holding that isn't regularly' is the wordplay.
'coming in' becomes 'entering' ('enter' can be a synonym of 'come in'**).
'holding' indicates putting letters inside.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'that'+'isnt'='thatisnt'
The alternate letters of 'thatisnt' are 'tain'.
'entering' placed around 'tain' is 'ENTERTAINING'.
(Other definitions for entertaining that I've seen before include "Being host - promoting enjoyment" , "fun" , "Comical" , "creating a diversion" , "Providing with amusement" .)