Coming down rapidly, thousands flee Reading in haste (6)
I believe the answer is:
skiing
'coming down rapidly' is the definition.
(eg skiing down a mountain)
'thousands flee reading in haste' is the wordplay.
'thousands' becomes 'mm' (Roman numerals).
'flee' suggests deleting specific letters (some letters flee or run away).
'reading in haste' becomes 'skimming' (to skim is to read quickly or hastily).
'skimming' with 'mm' removed is 'SKIING'.
(Other definitions for skiing that I've seen before include "Partaking in winter sport" , "Travelling over snow on runners" , "Participating in a winter sport" , "going downhill fast" , "Winter sports" .)