Bit of fun always bringing a high temperature (5)
I believe the answer is:
fever
'a high temperature' is the definition.
(I've seen this before)
'bit of fun always' is the wordplay.
'bit of' says to take the initial letters.
'always' becomes 'ever' ('ever' can be a synonym of 'always').
The first letter of 'fun' is 'f'.
'f'+'ever'='FEVER'
'bringing' is the link.
(Other definitions for fever that I've seen before include "Elvis hit" , "State of high excitement" , "State of heightened excitement" , "Ague, abnormally high body temperature" , "A rise in the temperature of the body" .)