Unexpected danger parking the thing before autumn (7)
I believe the answer is:
pitfall
'unexpected danger' is the definition.
(I've seen this before)
'parking the thing before autumn' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'the thing' becomes 'it'.
'before' says to put letters next to each other.
'autumn' becomes 'fall' (synonyms).
'p'+'it'+'fall'='PITFALL'
(Other definitions for pitfall that I've seen before include "Unexpected problem" , "Trap in the form of a concealed hole" , "Unsuspected danger or difficulty - for miner?" , "Unexpected difficulty or danger" , "Unforeseen difficulty" .)