Thorium ingested by a certain patient man working (2,3,3)
I believe the answer is:
on the job
'working' is the definition.
('on the job' can be a synonym of 'working')
'thorium ingested by a certain patient man' is the wordplay.
'thorium' becomes 'th' ('Th' can be a synonym of 'thorium').
'ingested by' indicates putting letters inside.
'a certain' becomes 'one' (I've seen this before).
'patient man' becomes 'job' (biblical character famous for patience**).
'th' inserted within 'one' is 'onthe'.
'onthe'+'job'='ON THE JOB'
(Other definitions for on the job that I've seen before include "At work" , "(Of training) done while working" , "Informally working, being busy" , "Bonking" .)