Key to Subaru initially found in aisle ten, perhaps (9)
I believe the answer is:
essential
'key' is the definition.
(I've seen this before)
'subaru initially found in aisle ten perhaps' is the wordplay.
'initially' indicates taking the first letters.
'found in' means one lot of letters goes inside another.
'perhaps' is an anagram indicator.
The first letter of 'subaru' is 's'.
'aisle'+'ten'='aisleten'
'aisleten' with letters rearranged gives 'esential'.
's' placed inside 'esential' is 'ESSENTIAL'.
'to' acts as a link.
(Other definitions for essential that I've seen before include "Basic or necessary" , "absolutely neccessary?" , "something necessary" , "Vital" , "must-have item" .)