In charge, one head showing hostility (7)
I believe the answer is:
iciness
'hostility' is the definition.
(I've seen this before)
'in charge one head' is the wordplay.
'in charge' becomes 'ic'.
'one' becomes 'i' (Roman numeral).
'head' becomes 'ness' (ness is a headland).
'ic'+'i'+'ness'='ICINESS'
'showing' is the link.
(Other definitions for iciness that I've seen before include "Extreme cold" , "Frozen state of" , "unfriendly manner" , "Extreme hostility" , "Hostile or cold nature" .)