Word in email regularly seen in plain language (7)
I believe the answer is:
promise
'word' is the definition.
(thesaurus)
'email regularly seen in plain language' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'seen in' means one lot of letters goes inside another.
'plain language' becomes 'prose' (I've seen this in other clues).
The alternate letters of 'email' are 'mi'.
'mi' put within 'prose' is 'PROMISE'.
'in' is the link.
(Other definitions for promise that I've seen before include "Hope; commitment" , "Hopeful sign" , "Give one's word" , "Assurance" , "Potential excellence" .)