Head of mouser, say, or bushy tail (8)
I believe the answer is:
category
'head' is the definition.
(thesaurus)
'mouser say or bushy tail' is the wordplay.
'mouser' becomes 'cat' (I've seen this in another clue).
'say' becomes 'eg' (both can mean 'for example').
'tail' says to take the final letters.
The final letter of 'bushy' is 'y'.
'cat'+'eg'+'or'+'y'='CATEGORY'
'of' acts as a link.
(Other definitions for category that I've seen before include "Classification of grey-coat" , "Class of grey-coat" , "Class of grey coat" , "Kind" , "Class with the same attribute" .)