Release when king takes charge (4)
I believe the answer is:
free
'release' is the definition.
(synonyms)
'king takes charge' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'takes' indicates putting letters inside (take can mean to capture or invade).
'charge' becomes 'fee' (fee is a kind of charge).
'r' going within 'fee' is 'FREE'.
'when' is the link.
(Other definitions for free that I've seen before include "At liberty" , "Unrestricted" , "Not confined" , "unemployed" , "As cheap as can be!" .)