Mature, since working for model employers (8)
I believe the answer is:
agencies
'model employers' is the definition.
(I've seen this in another clue)
'mature since working' is the wordplay.
'mature' becomes 'age' (synonyms).
'working' indicates anagramming the letters.
'since' is an anagram of 'ncies'.
'age'+'ncies'='AGENCIES'
'for' acts as a link.
(Other definitions for agencies that I've seen before include "Businesses that provide services on behalf of others" , "Organisations providing particular services" , "Businesses that serve others" , "Bureaux" , "Firms who transact business for others, perhaps actors" .)