Bit of fun with pressure line (5)
I believe the answer is:
prank
'bit of fun' is the definition.
(practical joke)
'with pressure line' is the wordplay.
'with' is a charade indicator (letters next to each other).
'pressure' becomes 'p' (scientific abbreviation).
'line' becomes 'rank' (rank is a kind of line).
'p'+'rank' is 'PRANK'.
(Other definitions for prank that I've seen before include "Practical joke or trick" , "Mischievous act done for fun and amusement" , "Lark" , "(Amusing) trick" , "April fool" .)