Engineers opening plans for cathedral (8)
I believe the answer is:
chartres
'cathedral' is the definition.
(French cathedral)
'engineers opening plans' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'opening' is an insertion indicator (some letters open a gap in others).
'plans' becomes 'charts' (charting is a kind of planning**).
're' put inside 'charts' is 'CHARTRES'.
'for' is the link.
(Other definitions for chartres that I've seen before include "Famous French cathedral city" , "French city - her carts (anag)" , "Cathedral city in north-west France" , "French town noted for its Gothic cathedral" , "French city with a cathedral renowned for its stained glass windows" .)