Pilot officer Muirhead's dog? (3)
I believe the answer is:
pom
'dog?' is the definition.
(short for Pomeranian)
'pilot officer muirhead's' is the wordplay.
'pilot officer' becomes 'po' (short for pilot officer).
'muirhead' becomes 'm' (head letter of 'muir').
'po'+'m'='POM'
(Other definitions for pom that I've seen before include "Englishman in Australia" , "Shortened name for a breed of dog" , "Australian slang for British migrant" , "Brit abroad" , "English person" .)