noun-modifying (irregular)
experienced; skilled; proficient
Used prenominally to describe a person with considerable skill or experience in a particular field.
彼は熟練した職人だ。
He is a skilled craftsman.
熟練した技術者を募集しています。
We are looking for experienced engineers.
From the noun 熟練 (じゅくれん, 'skill, proficiency') combined with the past tense of する, functioning as a prenominal adjective.