Toddler following small bullock (4)
I believe the answer is:
stot
'bullock' is the definition.
(I've seen this before)
'toddler following small' is the wordplay.
'toddler' becomes 'tot' ('tot' can be a synonym of 'toddler').
'following' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'tot' put after 's' is 'STOT'.
(Other definitions for stot that I've seen before include "Steer" , "Bullock" , "walk unsteadily in Glasgow" , "animal on farm" , "Sort of ox" .)