Alliance placing silver urn originally in shelter (6)
I believe the answer is:
league
'alliance' is the definition.
(I've seen this before)
'silver urn originally in shelter' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'originally' indicates taking the first letters.
'in' indicates putting letters inside.
'shelter' becomes 'lee' (as in lee side, the side sheltered from wind).
The initial letter of 'urn' is 'u'.
'ag'+'u'='agu'
'agu' put within 'lee' is 'LEAGUE'.
'placing' acts as a link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for league that I've seen before include "Group of clubs; distance" , "Association - old measure of distance" , "About five kilometres" , "Confederacy" , "Alliance, association" .)