Agree new head shows enthusiasm (9)
I believe the answer is:
eagerness
'enthusiasm' is the definition.
(eagerness is a kind of enthusiasm)
'agree new head' is the wordplay.
'new' indicates anagramming the letters.
'head' becomes 'ness' (ness is a headland).
'agree' is an anagram of 'eager'.
'eager'+'ness'='EAGERNESS'
'shows' acts as a link.
(Other definitions for eagerness that I've seen before include "Keen interest" , "Keenness, enthusiasm" , "Avidity" , "Sense rage (anag.)" , "'Keenness, avidness (9)'" .)