Smallest of small cars failing to start during trial (7)
I believe the answer is:
tiniest
'smallest' is the definition.
('tiny' can be similar in meaning to 'small')
'small cars failing to start during trial' is the wordplay.
'small cars' becomes 'mini' (eg Mini Cooper).
'failing to start' means to remove the first letter.
'during' means one lot of letters goes inside another.
'trial' becomes 'test' (synonyms).
'mini' with its initial letter taken away is 'ini'.
'ini' going within 'test' is 'TINIEST'.
'of' is the link.
(Other definitions for tiniest that I've seen before include "I sent it, being the smallest" , "Most small" .)