Purge one recalled as the worst time (5)
I believe the answer is:
nadir
'the worst time' is the definition.
(the lowest or worst period)
'purge one recalled' is the wordplay.
'purge' becomes 'rid' (as in ridding or purging somewhere of something).
'one' becomes 'an' ('a' and 'an' can mean 'one').
'recalled' says the letters should be written in reverse.
'rid'+'an'='ridan'
'ridan' written backwards gives 'NADIR'.
'as' is the link.
(Other definitions for nadir that I've seen before include "Very bottom, farthest from zenith" , "Low spot" , "Lower one cannot get!" , "Dinar (anag.)" , "Point opposite the zenith" .)