Accumulate in period prior to event (3,2)
I believe the answer is:
run up
This clue is a double definition.
'accumulate' is the first definition.
(run up can mean to accumulate or add up)
'period prior to event' is the second definition.
(the run up to something is the period preceding it)
'in' is the link.
(Other definitions for run up that I've seen before include "Bowler's approach" , "Cricketing approach?" , "formed quickly" , "Hoist" , "Period preceding a notable event" .)