Tether put on sheep in poor condition and likely to collapse (10)
I believe the answer is:
ramshackle
'in poor condition and likely to collapse' is the definition.
'tether put on sheep' is the wordplay.
'tether' becomes 'shackle'.
'put on' means one lot of letters go next to another.
'sheep' becomes 'ram' (ram is a kind of sheep).
'shackle' after 'ram' is 'RAMSHACKLE'.
(Other definitions for ramshackle that I've seen before include "falling apart" , "Badly-organised" , "In severe disrepair" , "falling to pieces" , "Decrepit" .)