Umpire trading iron for titanium — now he no longer works (7)
I believe the answer is:
retiree
'now he no longer works' is the definition.
(someone who has retired)
'umpire trading iron for titanium' is the wordplay.
I cannot really understand how this works, but
'titanium' could be 'ti' (Ti is the chemical symbol for titanium) and 'ti' is located in the answer.
This explanation may well be incorrect...
(Other definitions for retiree that I've seen before include "One stopping work" , "One no longer working" , "One put out to pasture" , "Person out of work" , "Someone no longer working" .)