Itching in middle of fleas: one getting on head (9)
I believe the answer is:
eagerness
'itching' is the definition.
(itching to do something)
'in middle of fleas one getting on head' is the wordplay.
'in middle of' indicates the central letters (I've seen 'in middle' mean this).
'one getting on' becomes 'ager' (someone aging or getting on a bit).
'head' becomes 'ness' (ness is a headland).
The middle letter of 'fleas' is 'e'.
'e'+'ager'+'ness'='EAGERNESS'
(Other definitions for eagerness that I've seen before include "Keen interest" , "'Keenness, avidness (9)'" , "Keen attitude" , "Sense rage (anag.)" , "Keenness, enthusiasm" .)