Royal Engineer regalia in industrial plant (8)
I believe the answer is:
refinery
'industrial plant' is the definition.
(I know that refinery is a type of industrial plant)
'royal engineer regalia' is the wordplay.
'royal engineer' becomes 'RE' (abbreviation).
'regalia' becomes 'finery' (synonyms).
're'+'finery'='REFINERY'
'in' is the link.
(Other definitions for refinery that I've seen before include "Factory which removes impurities etc from crude substances" , "Industrial plant for purifying crude substances" , "Purification plant" , "Plant for purifying, oil say" , "Place for improving crude material" .)