Whistle excessively strident, ultimately (4)
I believe the answer is:
toot
'whistle' is the definition.
(I've seen this in another clue)
'excessively strident ultimately' is the wordplay.
'excessively' becomes 'too' (to too great a degree).
'ultimately' suggests the final letters.
The last letter of 'strident' is 't'.
'too'+'t'='TOOT'
(Other definitions for toot that I've seen before include "Sound of a car horn" , "Do it on a flute" , "Briefly blow" , "Brief blast on a horn" , "Noise made by car horn" .)