At work, Kath itches to retire (3,3,4)
I believe the answer is:
hit the sack
'retire' is the definition.
(synonyms)
'at work kath itches' is the wordplay.
'at work' is an anagram indicator.
'kath'+'itches'='kathitches'
'kathitches' is an anagram of 'HIT THE SACK'.
'to' acts as a link.
(Other definitions for hit the sack that I've seen before include "Drank a lot of vintage wine" , "Go to bed" , "Turn in" , "retire" , "Retired" .)