Need one in charge when head is sacked (6)
I believe the answer is:
entail
'need' is the definition.
(entail means to require or give rise to)
'one in charge when head is sacked' is the wordplay.
'one' becomes 'I' (Roman numeral).
'in' indicates putting letters inside.
'charge' becomes 'rental' (the charge for renting something).
'when head is sacked' means to remove the first letter (the head of the word is removed or sacked).
'rental' with its first letter taken away is 'ental'.
'i' placed into 'ental' is 'ENTAIL'.
(Other definitions for entail that I've seen before include "settlement of estate" , "Cut" , "Restriction on inheritance" , "legal restriction" , "Bring on an inevitable consequence" .)