Observe duck head (3)
I believe the answer is:
loo
'head' is the definition.
(thesaurus)
'observe duck' is the wordplay.
'observe' becomes 'lo' (archaic exclamation meaning 'look!').
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'lo'+'o'='LOO'
(Other definitions for loo that I've seen before include "Card game; the 'smallest room'" , "A colloquial toilet" , "Rest room" , "bog" , "WC - old card game" .)