One beginning gallop energetically, but not all (6)
I believe the answer is:
opener
'one beginning' is the definition.
(opener is a kind of beginning)
'gallop energetically but not all' is the wordplay.
'not all' says the answer is hidden in the clue.
'OPENER' is hidden within 'gallop energetically but'.
(Other definitions for opener that I've seen before include "player, first to bat" , "Device for accessing contents of tins or bottles" , "initial goal" , "man, first at the crease?" , "(Team's) first batsman" .)