You can't say he doesn't give a toot! (4)
I believe the answer is:
'you can't say he doesn't' is the definition.
The definition suggests a singular noun which matches the answer.
'give a toot' is the wordplay.
'give' indicates anagramming the letters (I've seen 'giving' mean this (the letters give the letters for another word)).
'toot' with letters rearranged gives 'OTTO'.
Can you help me to learn more?
(Other definitions for otto that I've seen before include "fragrance comes from?" , "Four-stroke cycle -- palindromic name" , "that smells quite nice" , "he goes" , "A man" .)