Hundreds initially fleeing icy enclosure in autumn, so to speak (3,3)
I believe the answer is:
old age
'autumn so to speak' is the definition.
(old age is the 'autumn' of someone's life)
I can't explain the remainder of the clue.
(Other definitions for old age that I've seen before include "State of very senior citizens" , "Pensionable condition" , "time of one's life!" , "Lego ad (anag)" , "Advanced years" .)