Stick heard shattering — ultimately brittle (6)
I believe the answer is:
adhere
'stick' is the definition.
('adhere' can be a synonym of 'stick')
'heard shattering ultimately brittle' is the wordplay.
'shattering' indicates an anagram.
'ultimately' indicates one should take the final letters.
The final letter of 'brittle' is 'e'.
'heard' is an anagram of 'adher'.
'adher'+'e'='ADHERE'
(Other definitions for adhere that I've seen before include "Stick to - cement" , "Agree" , "Bond" , "Stick to, remain loyal" , "Hold fast, stand by" .)