Trouble in the past overwhelming centre of Zagreb (5)
I believe the answer is:
aggro
'trouble' is the definition.
(short for aggression or aggravation)
'past overwhelming centre of zagreb' is the wordplay.
'past' becomes 'ago' (similar in meaning).
'overwhelming' indicates putting letters inside.
'centre of' says to take the centre.
The centre of 'zagreb' is 'gr'.
'ago' placed around 'gr' is 'AGGRO'.
'in the' acts as a link.
(Other definitions for aggro that I've seen before include "Fighting" , "Antisocial behaviour" , "Boorish, threatening behaviour (slang)" , "Bovver!" , "offensive behaviour" .)