Endlessly slow, turning up? Blast! (4)
I believe the answer is:
drat
'blast' is the definition.
(I've seen this before)
'endlessly slow turning up?' is the wordplay.
'endlessly' means to remove the last letter.
'slow' becomes 'tardy' (tardy means running late).
'turning up?' is a reversal indicator.
'tardy' with its final letter taken away is 'tard'.
'tard' back-to-front is 'DRAT'.
(Other definitions for drat that I've seen before include "blast" , "Darn it!" , "Informal expression of annoyance" , "Minced oath" , "Curse it!" .)