Court case mostly involved military vehicle (5,3)
I believe the answer is:
scout car
'military vehicle' is the definition.
(scout car is a kind of military vehicle)
'court case mostly involved' is the wordplay.
'mostly' means to remove the last letter.
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'court'+'case'='courtcase'
'courtcase' with its last letter taken away is 'courtcas'.
'courtcas' is an anagram of 'SCOUT CAR'.
(Other definitions for scout car that I've seen before include "Fast armoured vehicle" , "reconnaissance vehicle" , "Military patroller" .)