Member starts with Theory of Everything (3)
I believe the answer is:
toe
'member' is the definition.
(I know that toe is a type of Dactyls)
'starts with theory of everything' is the wordplay.
'starts with' suggests taking the first letters.
The first letters of 'theory of everything' is 'TOE'.
(Other definitions for toe that I've seen before include "Digit on foot" , "Foot extremity." , "compliantly put on line" , "End of foot" , "One with sock on" .)