Letter of introduction lad in Crete recycled (10)
I believe the answer is:
credential
'letter of introduction' is the definition.
(thesaurus)
'lad in crete recycled' is the wordplay.
'recycled' indicates anagramming the letters.
'lad'+'in'+'crete'='ladincrete'
'ladincrete' anagrammed gives 'CREDENTIAL'.
(Other definitions for credential that I've seen before include "Qualification (indicating a person's suitability)" , "Interlaced (anag.)" , "Official introductory letter" , "proof of entitlement" .)