Power held by very severe overseas correspondent (3,3)
I believe the answer is:
pen pal
'overseas correspondent' is the definition.
(I've seen this before)
'power held by very severe' is the wordplay.
'power' becomes 'p' (symbol in physics).
'held by' is an insertion indicator.
'very severe' becomes 'penal' (I've seen this before).
'p' put inside 'penal' is 'PEN-PAL'.
(Other definitions for pen pal that I've seen before include "juvenile writer" , "American friend via correspondence" , "A friend who communicates through correspondence" , "friendly writer" , "Friend who communicates in letters" .)