In trouble mostly due to brief prologue (5)
I believe the answer is:
intro
'prologue' is the definition.
(I've seen this in another clue)
'in trouble mostly due to brief' is the wordplay.
I cannot quite see how this works, but
'in' is within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for intro that I've seen before include "First bars of a musical composition" , "Opening bars to a piece of popular music" , "Lead-in" , "starter" , "Preliminary part" .)