I head Unity (7)
I believe the answer is:
oneness
'unity' is the definition.
(I know that unity can be written as oneness)
'i head' is the wordplay.
'i' becomes 'one' (Roman numeral for one).
'head' becomes 'ness' (ness is a headland).
'one'+'ness'='ONENESS'
(Other definitions for oneness that I've seen before include "Unity" , "Agreement" , "Identity" , "#Unique quality" , "singularity" .)