Element unknown in plant (7)
I believe the answer is:
factory
'plant' is the definition.
(I know that factory is a type of plant)
'element unknown' is the wordplay.
I cannot quite see how this works, but
'element' could be 'factor' (synonyms) and 'factor' is located in the answer.
'unknown' could be 'y' (algebra) and 'y' is found in the answer.
No letters remain.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for factory that I've seen before include "Manufacturing plant" , "Place for production" , "Works" , "mill" , "Building used for making or assembling items" .)