Finally sag — not tall — crumbling (2,4,4)
I believe the answer is:
at long last
'finally' is the definition.
(synonyms)
'sag not tall crumbling' is the wordplay.
'crumbling' is an anagram indicator.
'sag'+'not'+'tall'='sagnottall'
'sagnottall' is an anagram of 'AT LONG LAST'.
(Other definitions for at long last that I've seen before include "Finally" , "After much delay" , "producing larger shoes?" , "cobbling size seventeens?" , "Eventually" .)