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' says to put letters next to each other.
'pressure' becomes 'p' (abbreviation).
'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 "Practical joke or trick" , "Mischievous act done for fun and amusement" , "Lark" , "(Amusing) trick" , "April fool" .)