Maiden as Donald's opening over mixed seam with length (6)
I believe the answer is:
damsel
'maiden' is the definition.
(I've seen this before)
'donald's opening over mixed seam with length' is the wordplay.
'opening' says to take the initial letters.
'over' means one lot of letters go next to another.
'mixed' is an anagram indicator.
'with' means one lot of letters go next to another.
'length' becomes 'l'.
The first letter of 'donald' is 'd'.
'seam' with letters rearranged gives 'amse'.
'd'+'amse'+'l'='DAMSEL'
'as' is the link.
(Other definitions for damsel that I've seen before include "Maiden (poetic)" , "Girl - type of fly" , "little fly" , "Young woman (archaic)" , "Maiden, often in distress" .)