Before devout lady losing heart, minister installed (10)
I believe the answer is:
Here is my best explanation:
'before' is the definition.
('previously' can be a synonym of 'before')
'heart minister installed' is the wordplay.
'heart' becomes 'piously' (I can't justify this - if you can you should give a lot more credence to this answer).
'minister' becomes 'rev' (I've seen this before).
'installed' means one lot of letters goes inside another.
'piously' going around 'rev' is 'PREVIOUSLY'.
'devout lady losing' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.