A pot initially concealed small garden pests (6)
I believe the answer is:
aphids
'garden pests' is the definition.
(I've seen this in another clue)
'a pot initially concealed small' is the wordplay.
'initially' says to take the initial letters.
'concealed' becomes 'hid' ('hide' can be a synonym of 'conceal').
'small' becomes 's' (abbreviation - e.g. clothes size).
The first letter of 'pot' is 'p'.
'a'+'p'+'hid'+'s'='APHIDS'
(Other definitions for aphids that I've seen before include "Plant pests" , "Sucking insects" , "Small plant-sucking insects" , "Garden pests" , "Plant lice like greenfly" .)