A long time and a bad one (4)
I believe the answer is:
aeon
'a long time' is the definition.
(aeon is a kind of long time)
'a bad one' is the wordplay.
'bad' indicates anagramming the letters.
'one' is an anagram of 'eon'.
'a'+'eon'='AEON'
'and' is the link.
(Other definitions for aeon that I've seen before include "Immeasurably long period of time" , "Very long and indefinite period of time" , "Longest division of geological time" , "Vast age" , "long term" .)