Retiring late is holding it up (8)
I believe the answer is:
reticent
'retiring' is the definition.
(thesaurus)
'late is holding it up' is the wordplay.
'late' becomes 'recent' ('recent' can be similar in meaning to 'late'**).
'is holding' indicates putting letters inside.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'it' written backwards gives 'ti'.
'recent' placed around 'ti' is 'RETICENT'.
(Other definitions for reticent that I've seen before include "Reserved, shy" , "Not wanting to talk" , "Disinclined to speak" , "Not revealing one's thoughts readily" , "Quiet" .)