… Agnetha performing in English version of Mamma Mia dress on this? (10)
I believe the answer is:
coathanger
'dress on this?' is the definition.
(a dress might be hung on a coathanger)
'agnetha performing in english version of mamma mia' is the wordplay.
'performing' is an anagram indicator.
'in' indicates putting letters inside.
'english version of mamma mia' becomes 'cor' (surprised expression similar to with 'mamma mia').
'agnetha' anagrammed gives 'athange'.
'athange' put inside 'cor' is 'COATHANGER'.
(Another definition for coathanger that I've seen is " Curved piece of wood with a hook in a wardrobe".)