Hold back a bit of breakfast, if leaving (6)
I believe the answer is:
stifle
'hold back' is the definition.
(thesaurus)
'a bit of breakfast if leaving' is the wordplay.
'a bit of' says the answer is hidden in the clue (I've seen 'a bit' mean this (the answer is formed by a bit of the clue)).
'STIFLE' can be found hidden inside 'breakfast if leaving'.
(Other definitions for stifle that I've seen before include "Smother - 6 down (anag)" , "Joint in hind leg of horse" , "Suppress of choke" , "Smother or suppress a feeling or action" , "Repress, choke" .)