One in ten's maybe a mug (5)
I believe the answer is:
stein
'a mug' is the definition.
(I know that stein is a type of mug)
'one in ten's maybe' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'maybe' indicates an anagram.
'tens' is an anagram of 'sten'.
'i' placed inside 'sten' is 'STEIN'.
(Other definitions for stein that I've seen before include "Earthenware beer jug" , "German tankard" , "Large earthenware beer mug" , "German earthenware beer mug" , "Beer-mug" .)