Chemical tester finding uranium in illuminated manuscript (6)
I believe the answer is:
litmus
'chemical tester' is the definition.
(I've seen this before)
'uranium in illuminated manuscript' is the wordplay.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'in' indicates putting letters inside.
'illuminated' becomes 'lit' (synonyms).
'manuscript' becomes 'ms' (abbreviation for manuscript).
'lit'+'ms'='litms'
'u' placed within 'litms' is 'LITMUS'.
'finding' acts as a link.
(Other definitions for litmus that I've seen before include "Colouring material used as a rough acid-base indicator" , "Old dance" , "Substance turned red by acids" , "It turns red in acid and blue in alkali" , "Acid-alkali testing substance" .)