Worker from agency yet to arrive — a pattern (8)
I believe the answer is:
template
'a pattern' is the definition.
(thesaurus)
'worker from agency yet to arrive' is the wordplay.
'worker from agency' becomes 'temp'.
'yet to arrive' becomes 'late'.
'temp'+'late'='TEMPLATE'
(Other definitions for template that I've seen before include "Prototype" , "Pattern or mould" , "Pattern shape" , "Model, example to follow" , "Pattern for document etc" .)