Mug? One can be seen in ultimately crass PM's address! (5)
I believe the answer is:
stein
'mug?' is the definition.
(stein is a kind of mug)
'one can be seen in ultimately crass pm's address' is the wordplay.
'one' becomes 'i' (Roman numeral).
'can be seen in' means one lot of letters goes inside another.
'ultimately' says to take the final letters.
'pm's address' becomes 'ten' (the prime minister lives in No i0 Downing Street).
The final letter of 'crass' is 's'.
's'+'ten'='sten'
'i' inserted inside 'sten' is 'STEIN'.
(Other definitions for stein that I've seen before include "Beer-mug" , "Large earthenware beer mug" , "Earthenware beer jug" , "German earthenware beer mug" , "Gertrude ---; US writer" .)