In which the Romans had been lying about starting trouble (5)
I believe the answer is:
latin
'in which the romans' is the definition.
Although both the answer and definition are adjectives, I can't understand how they can define each other.
'been lying about starting trouble' is the wordplay.
'been lying' becomes 'lain' (I am not sure about this - if you are sure you should believe this answer much more).
'about' is an insertion indicator.
'starting' indicates taking the first letters.
The first letter of 'trouble' is 't'.
'lain' enclosing 't' is 'LATIN'.
'had' acts as a link.
Can you help me to learn more?
(Other definitions for latin that I've seen before include "foreign language" , "and 15 Down: Student area of Paris" , "Shakespeare's was small, according to Jonson" , "Old Roman language" , "language class" .)