Agree with head of Cruft's about dog (6)
I believe the answer is:
concur
'agree with' is the definition.
(thesaurus)
'head of cruft's about dog' is the wordplay.
'head of' suggests taking the first letters.
'about' becomes 'on'.
'dog' becomes 'cur' (cur is a kind of dog).
The first letter of 'cruft' is 'c'.
'c'+'on'+'cur'='CONCUR'
(Other definitions for concur that I've seen before include "Stone pile" , "'Agree, coincide (6)'" , "Give assent" , "run together" , "Agree, assent" .)