Liven up a part of the UK with chum (7)
I believe the answer is:
animate
'liven up' is the definition.
('animate' can be a synonym of 'liven up')
'a part of the uk with chum' is the wordplay.
'part of the uk' becomes 'ni' (Northern Ireland).
'with' means one lot of letters go next to another.
'chum' becomes 'mate' (I've seen this before).
'a'+'ni'+'mate'='ANIMATE'
(Other definitions for animate that I've seen before include "Fire" , "Give inspiration to" , "Give new life or energy to" , "Invigorate; alive" , "Eat main (anag.)" .)