Venetian leader receives thanks in declining years (6)
I believe the answer is:
dotage
'declining years' is the definition.
'venetian leader receives thanks' is the wordplay.
'venetian leader' becomes 'doge' (I've seen this before).
'receives' is an insertion indicator.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'doge' going around 'ta' is 'DOTAGE'.
'in' acts as a link.
(Other definitions for dotage that I've seen before include "when any activity may become difficult?" , "Feeble-minded period late in life" , "In which one would be old and weak" , "Feeble later life" , "Old age, senility" .)