Suggest one will be dressed in mail (5)
I believe the answer is:
posit
'suggest' is the definition.
(thesaurus)
'one will be dressed in mail' is the wordplay.
'one will be' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'dressed in' indicates putting letters inside (inserted letters are dressed in others).
'mail' becomes 'post' (synonyms).
'i' inserted inside 'post' is 'POSIT'.
Can you help me to learn more?
(Other definitions for posit that I've seen before include "given proposition" , "Assume as fact for argument's sake" , "Propose" , "Assume something as a basis for discussion" , "Put forward (argument)" .)