About to leave oil production plant in glad rags (6)
I believe the answer is:
finery
'glad rags' is the definition.
(I've seen this in another clue)
'about to leave oil production plant in' is the wordplay.
I cannot really see how this works, but
'in' is within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for finery that I've seen before include "Glad rags, best duds" , "Fancy clothes or trappings" , "Ostentatious decoration" , "Ostentatious clothes" , "Elaborate clothing, jewellery etc" .)