One is late (not early): only some changes holding delivery up (12)
I believe the answer is:
monosyllable
'one is late not early' is the definition.
('late' has one syllable)
'only some changes holding delivery up' is the wordplay.
'changes' indicates anagramming the letters.
'holding' means one lot of letters goes inside another.
'delivery' becomes 'ball' (ball is a kind of delivery).
'up' is a reversal indicator (in down clue: letters go upwards).
'ball' in reverse letter order is 'llab'.
'only'+'some'='onlysome'
'onlysome' is an anagram of 'monosyle'.
'monosyle' placed around 'llab' is 'MONOSYLLABLE'.
(Other definitions for monosyllable that I've seen before include "Word of one sound unit" , "Briefest possible utterance" , "Word such as "yes" or "no"" , "Short word, such as "short" or "word"" , "Moll's baloney (anag.)" .)