Teacher unthinkingly grabbing smallest of the lot (4)
I believe the answer is:
runt
'smallest of the lot' is the definition.
(as in the runt of the litter)
'teacher unthinkingly grabbing' is the wordplay.
'grabbing' indicates the answer is hidden within the clue (the answer is held or grabbed by the clue).
'RUNT' can be found hidden inside 'teacher unthinkingly'.
(Other definitions for runt that I've seen before include "little one" , "Weak individual" , "Smallest squealer" , "Smallest animal in a litter" , "little piggy" .)