Leaders of our Liberal Democrats, advanced in years (3)
I believe the answer is:
old
'advanced in years' is the definition.
(I've seen this in other clues)
'leaders of our liberal democrats' is the wordplay.
'leaders of' indicates taking the first letters.
The initial letters of 'our liberal democrats' is 'OLD'.
(Other definitions for old that I've seen before include "Getting on" , "Long in the tooth" , "Wild clematis" , "No longer young" , "Venerable" .)