Attendant put on petticoat or an unwanted shift (8)
I believe the answer is:
slippage
'an unwanted shift' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'attendant put on petticoat' is the wordplay.
'attendant' becomes 'page' (page is a kind of attendant).
'put on' is a charade indicator (letters next to each other).
'petticoat' becomes 'slip' (I've seen this in another clue).
'page' after 'slip' is 'SLIPPAGE'.
'or' is the link.
Can you help me to learn more?
(Other definitions for slippage that I've seen before include "Lag pipes (anag)" , "falling away from a standard" , "sliding" , "Failure to reach a set target" , "Failure to meet target" .)