One worked for period (3)
I believe the answer is:
eon
'period' is the definition.
(eon is a kind of period)
'one worked' is the wordplay.
'worked' indicates anagramming the letters.
'one' anagrammed gives 'EON'.
'for' is the link.
(Other definitions for eon that I've seen before include "Age, epoch" , "In time, long division" , "Very long period (US and technical spelling)" , "Prolonged period" , "Geological period" .)