Plug small cavity, full of energy (6)
I believe the answer is:
spigot
'plug' is the definition.
(spigot is a kind of plug)
'small cavity full of energy' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cavity' becomes 'pit' ('pit' can be a synonym of 'cavity').
'full of' is an insertion indicator.
'energy' becomes 'go' (I've seen this in other clues**).
'pit' enclosing 'go' is 'pigot'.
's'+'pigot'='SPIGOT'
(Other definitions for spigot that I've seen before include "It's a water tap in America" , "Peg that stops the vent of a cask" , "One stops" , "Tap, faucet" , "Peg for a cask's vent" .)