Shabby mandate, mostly mine (8)
I believe the answer is:
decrepit
'shabby' is the definition.
(thesaurus)
'mandate mostly mine' is the wordplay.
'mandate' becomes 'decree' (I've seen this in another clue).
'mostly' means to remove the last letter.
'mine' becomes 'pit' (down the pit).
'decree' with its final letter taken away is 'decre'.
'decre'+'pit'='DECREPIT'
(Other definitions for decrepit that I've seen before include "Worn-out" , "run down" , "ruined by neglect" , "like a crate?" , "Dilapidated, made weak by age" .)