Suggest one wears mail (5)
I believe the answer is:
posit
'suggest' is the definition.
(I've seen this before)
'one wears mail' is the wordplay.
'one' becomes 'i' (Roman numeral).
'wears' indicates putting letters inside.
'mail' becomes 'post' (I've seen this before).
'i' placed inside 'post' is 'POSIT'.
(Other definitions for posit that I've seen before include "Assume something as a basis for discussion" , "Assume as fact, postulate" , "Suggest" , "Lay down as principle" , "Put forward as basis for discussion" .)