Trial started badly, imprisoning first of offenders (4,4)
I believe the answer is:
road test
'trial' is the definition.
(road test is a kind of trial)
'started badly imprisoning first of offenders' is the wordplay.
'badly' indicates anagramming the letters.
'imprisoning' means one lot of letters goes inside another.
'first of' says to take the initial letters.
The first letter of 'offenders' is 'o'.
'started' is an anagram of 'radtest'.
'radtest' going around 'o' is 'ROAD TEST'.
(Other definitions for road test that I've seen before include "critical examination of product" , "check car" , "preliminary trial" , "Start ode (anag.)" , "Challenge for car" .)