In brief, nothing but the royal train (5)
I believe the answer is:
court
'the royal train' is the definition.
'court' can be an answer for 'train' (thesaurus). I'm unsure of the 'the royal' bit.
'in brief nothing' is the wordplay.
'in' is an insertion indicator.
'brief' becomes 'curt' (synonyms).
'nothing' becomes 'O' (looks like zero - 0).
'curt' going around 'o' is 'COURT'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for court that I've seen before include "Sports area; woo" , "Tennis arena" , "Judicial body" , "SECOND PART OF 5" , "Tennis playing area" .)