Last of owls has best hoot! (6)
I believe the answer is:
scream
'hoot' is the definition.
(thesaurus)
'last of owls has best' is the wordplay.
'last of' says to take the final letters.
'has' says to put letters next to each other.
'best' becomes 'cream' (I've seen this before).
The last letter of 'owls' is 's'.
's'+'cream'='SCREAM'
(Other definitions for scream that I've seen before include "Hoot - yell in pain" , "Funny person; cry" , "A shrill cry of fright" , "funny man" , "irresistibly funny person" .)