Horse, somewhere in East Anglia, with striking power (7,5)
I believe the answer is:
suffolk punch
'horse' is the definition.
(I've seen this before)
'somewhere in east anglia with striking power' is the wordplay.
'somewhere in east anglia' becomes 'Suffolk' (Suffolk is a county in East Anglia).
'with' means one lot of letters go next to another.
'striking power' becomes 'punch' (I've seen this before).
'suffolk'+'punch'='SUFFOLK PUNCH'
(Other definitions for suffolk punch that I've seen before include "horse" , "Aldeburgh festival?" , "One helping to deliver drink" .)