Difficulties since introducing Greek (5)
I believe the answer is:
aggro
'difficulties' is the definition.
(I've seen this before)
'since introducing greek' is the wordplay.
'since' becomes 'ago' (I've seen this in another clue).
'introducing' means one lot of letters goes inside another (some letters introduced or inserted into others).
'greek' becomes 'gr' (abbreviation for Greek).
'ago' placed around 'gr' is 'AGGRO'.
(Other definitions for aggro that I've seen before include "Big trouble" , "Hostile troublemaking" , "In slang, hassle or troublemaking" , "Belligerent trouble" , "Difficulties, trouble" .)