Small number going back to the beginning of Edwardian age (3)
I believe the answer is:
eon
'age' is the definition.
(I know that eon is a type of age)
'small number going back to the beginning of edwardian' is the wordplay.
'small number' becomes 'no' (abbreviation for 'number'; 'small' suggests an abbreviation).
'going back' says the letters should be written in reverse.
'to' says to put letters next to each other (I've seen this in other clues).
'the beginning of' indicates taking the first letters.
The first letter of 'edwardian' is 'e'.
'no' reversed gives 'on'.
'on' put after 'e' is 'EON'.
(Other definitions for eon that I've seen before include "Very long age" , "Vast age; eternity" , "billion years" , "Age, epoch" , "thousand million years" .)