Bit of fun with power line (5)
I believe the answer is:
prank
'bit of fun' is the definition.
(practical joke)
'with power line' is the wordplay.
'with' says to put letters next to each other.
'power' becomes 'p' (symbol in physics).
'line' becomes 'rank' (rank is a kind of line).
'p' next to 'rank' is 'PRANK'.
(Other definitions for prank that I've seen before include "High-spirited mischief" , "Jape" , "(Amusing) trick" , "Escapade" , "Practical joke or trick" .)