Post Office trainees emptied containers (4)
I believe the answer is:
pots
'containers' is the definition.
(pot is a kind of container)
'post office trainees emptied' is the wordplay.
'post office' becomes 'po' (abbreviation).
'emptied' means to remove the middle letters.
'trainees' with its centre removed is 'ts'.
'po'+'ts'='POTS'
(Other definitions for pots that I've seen before include "earthenware here" , "Cooking utensils" , "Stop (anag.)" , "Containers; oodles" , "Preserves" .)