Sister is in ruins, every now and then (3)
I believe the answer is:
nun
'sister' is the definition.
(thesaurus)
'and then' is the wordplay.
'and' becomes 'n' (common abbreviation for 'and').
'then' becomes 'un' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'n'+'un'='NUN'
'is in ruins every now' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for nun that I've seen before include "Woman in Holy Orders" , "Fancy pigeon variety" , "Mother Teresa, maybe" , "Priory female" , "Joshua's father" .)