Match one head’s irritability (9)
I believe the answer is:
testiness
'irritability' is the definition.
(I know that testiness is a type of irritability)
'match one head's' is the wordplay.
'match' becomes 'testi' (I can't justify this - if you can you should believe this answer much more).
'one head' becomes 'ness' (ness is a headland).
'testi'+'ness'='TESTINESS'
(Other definitions for testiness that I've seen before include "Irritability" , "Irascibility" .)