Oppressed group of workers (excellent/no good?) in endless reorganisation (5,4)
I believe the answer is:
chain gang
'oppressed group of workers' is the definition.
(group of workers chained together)
'excellentno good? in endless reorganisation' is the wordplay.
'excellentno' becomes 'ain' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'good?' becomes 'g' (abbreviation).
'in' indicates putting letters inside.
'endless' means to remove the last letter.
'reorganisation' becomes 'change'.
'ain'+'g'='aing'
'change' with its final letter removed is 'chang'.
'aing' inserted inside 'chang' is 'CHAIN-GANG'.
(Other definitions for chain gang that I've seen before include "Manacled workers" , "Shackled workers?" , "Group of convicts coupled together while working" , "penal group" , "Linked prisoners" .)