Deployed in fresco, a tin-glazed veneer (7)
I believe the answer is:
coating
'veneer' is the definition.
(thesaurus)
'deployed in fresco a tin-glazed' is the wordplay.
'deployed in' says the answer is hidden in the clue.
'COATING' is hidden within 'fresco a tin glazed'.
(Other definitions for coating that I've seen before include "Protective layer" , "film" , "Thin layer; covering" .)