Warn of trouble converting theatre, Northern (8)
I believe the answer is:
threaten
'warn of trouble' is the definition.
'threaten' can be an answer for 'warn' (threatening is a kind of warning). I am not sure about the 'of trouble' bit.
'converting theatre northern' is the wordplay.
'converting' is an anagram indicator.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'theatre' is an anagram of 'threate'.
'threate'+'n'='THREATEN'
Can you help me to learn more?
(Other definitions for threaten that I've seen before include "Menace, warn of intention to harm" , "Put in jeopardy" , "Give warning of something unpleasant" , "'Menace, intimidate (8)'" , "Utter menaces" .)