Mark silver jubilee at last, showing excessive fondness (6)
I believe the answer is:
dotage
'excessive fondness' is the definition.
(I've seen this in another clue)
'mark silver jubilee at last' is the wordplay.
'mark' becomes 'dot' (dotting is a kind of marking).
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'at last' suggests the final letters.
The final letter of 'jubilee' is 'e'.
'dot'+'ag'+'e'='DOTAGE'
'showing' is the link.
(Other definitions for dotage that I've seen before include "Mental infirmity in old age" , "State of weakness of mind caused by old age" , "Old and weak time of life" , "period of twilight" , "Foolish infatuation" .)