Enthusiasm to agree changes with head (9)
I believe the answer is:
eagerness
'enthusiasm' is the definition.
(I know that eagerness is a type of enthusiasm)
'agree changes with head' is the wordplay.
'changes' indicates an anagram.
'with' says to put letters next to each other.
'head' becomes 'ness' (ness is a headland).
'agree' anagrammed gives 'eager'.
'eager'+'ness'='EAGERNESS'
'to' is the link.
(Other definitions for eagerness that I've seen before include "'Keenness, avidness (9)'" , "Keen attitude" , "Sense rage (anag.)" , "Keen interest" , "Avidity" .)