Sweetheart's overly feminine upping pace (8)
I believe the answer is:
footstep
'pace' is the definition.
(I know that pace can be written as footstep)
'sweetheart's overly feminine upping' is the wordplay.
'sweetheart's' becomes 'pet's' (both are terms of endearment).
'overly' becomes 'too' (to too great a degree).
'feminine' becomes 'f' (abbreviation listed in Chambers).
'upping' says the letters should be written backwards (I've seen 'upped' mean this (in down clue, made to go up)).
'pets'+'too'+'f'='petstoof'
'petstoof' backwards is 'FOOTSTEP'.
(Other definitions for footstep that I've seen before include "Mark made when walking" , "Sound of walker" , "Sound of a pace" , "Sound of a step of someone walking" .)