Scan one head in preparation (9)
I believe the answer is:
readiness
'preparation' is the definition.
(I know that preparation can be written as readiness)
'scan one head' is the wordplay.
'scan' becomes 'read' (synonyms).
'one' becomes 'i' (Roman numeral).
'head' becomes 'ness' (ness is a headland).
'read'+'i'+'ness'='READINESS'
'in' is the link.
(Other definitions for readiness that I've seen before include "Immediate availability" , "State of preparedness" , "Promptitude" , "Inclination" , "State of being prepared and willing" .)