Long time taken up securing pit candidate (7)
I believe the answer is:
nominee
'candidate' is the definition.
(I've seen this before)
'long time taken up securing pit' is the wordplay.
'long time' becomes 'eon' (eon is a kind of long time).
'taken up' says the letters should be written in reverse.
'securing' indicates putting letters inside.
'pit' becomes 'mine' (I've seen this before).
'eon' reversed gives 'noe'.
'noe' enclosing 'mine' is 'NOMINEE'.
(Other definitions for nominee that I've seen before include "No real owner" , "One put forward as a candidate" , "Person put up for a post" , "His/her name is put forward" , "Person put forward" .)