Threaten to scrap unreliable car failing to start (8)
I believe the answer is:
endanger
'threaten' is the definition.
(synonyms)
'scrap unreliable car failing to start' is the wordplay.
'scrap' becomes 'end' (synonyms).
'unreliable car' becomes 'banger' (I've seen this before).
'failing to start' means to remove the first letter.
'banger' with its first letter taken away is 'anger'.
'end'+'anger'='ENDANGER'
'to' acts as a link.
(Other definitions for endanger that I've seen before include "Jeopardize" , "Put in jeopardy" , "Place into peril, jeopardise" , "Dan Green (anag.)" , "To put one at risk" .)