Trailer parking needs formal assessment (7)
I believe the answer is:
preview
'trailer' is the definition.
(I've seen this before)
'parking needs formal assessment' is the wordplay.
'parking' becomes 'p'.
'needs' is a charade indicator (letters next to each other).
'formal assessment' becomes 'review' (I've seen this before).
'p'+'review'='PREVIEW'
(Other definitions for preview that I've seen before include "Foretaste" , "Hint" , "A showing before opening night" , "special showing" , "early show" .)