Trailer, one splitting near the bottom (5)
I believe the answer is:
nadir
'bottom' is the definition.
(I've seen this before)
'trailer one splitting near' is the wordplay.
'trailer' becomes 'ad' (short for advert).
'one' becomes 'i' (Roman numeral).
'splitting' indicates putting letters inside (some letters split a word into two).
'near' becomes 'nr' (abbreviation used in addresses).
'ad'+'i'='adi'
'adi' placed inside 'nr' is 'NADIR'.
'the' acts as a link.
(Other definitions for nadir that I've seen before include "deep despair" , "The lowest point of anything" , "Point opposite the zenith" , "Drain into the lowest point (5)" , "It's the depths" .)