Reluctance to move in time is holding it up (7)
I believe the answer is:
inertia
'reluctance to move' is the definition.
(I've seen this before)
'in time is holding it up' is the wordplay.
'time' becomes 'era' (I've seen this before**).
'is holding' is an insertion indicator.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'it' back-to-front is 'ti'.
'era' placed around 'ti' is 'ertia'.
'in'+'ertia'='INERTIA'
(Other definitions for inertia that I've seen before include "Lack of movement" , "Disinclination to change" , "Lethargy" , "Slowness to take action, state of laziness" , "Sort of reel" .)