Erratic teen's test drive is personal concern (6,8)
I believe the answer is:
vested interest
'personal concern' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'erratic teen's test drive' is the wordplay.
'erratic' indicates anagramming the letters.
'teens'+'test'+'drive'='teenstestdrive'
'teenstestdrive' is an anagram of 'VESTED INTEREST'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for vested interest that I've seen before include "It could be good for you!" , "Personal stake in an undertaking" , "Having personal concern" .)