Sound excessively thick-headed (4)
I believe the answer is:
'sound' is the definition.
(tooting is a kind of sounding)
'excessively thick-headed' is the wordplay.
'excessively' becomes 'too' (synonyms).
'thick-headed' becomes 't' (this could be a standard abbreviation which I don't know about).
Can you help me to learn more?
(Other definitions for toot that I've seen before include "Brief blast on a horn" , "Sound one's horn" , "Noise made by car horn" , "Do it on a flute" , "Briefly blow" .)