Power lines around area become dull (4)
I believe the answer is:
pall
'become dull' is the definition.
(I've seen this in another clue)
'power lines around area' is the wordplay.
'power' becomes 'p' (symbol in physics).
'lines' becomes 'll' (an abbreviation for line written twice).
'around' indicates putting letters inside.
'area' becomes 'a' (maths abbreviation).
'p'+'ll'='pll'
'pll' placed around 'a' is 'PALL'.
(Other definitions for pall that I've seen before include "Cloy" , "A dark cover; cease to interest" , "Become tiresome" , "Cloth to cover a coffin" , "Become less interesting" .)