Embryonic sac in mother inverted in operation (6)
I believe the answer is:
amnion
'embryonic sac' is the definition.
(I've seen this in another clue)
'in mother inverted in operation' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'inverted' is a reversal indicator.
'in operation' becomes 'on'.
'in'+'ma'='inma'
'inma' reversed gives 'amni'.
'amni'+'on'='AMNION'
(Other definitions for amnion that I've seen before include "Innermost membrane" , "Film about site of early growth" , "One embryo membrane" .)