Is a glue not unusually viscous? (10)
I believe the answer is:
gelatinous
'viscous?' is the definition.
(thesaurus)
'is a glue not unusually' is the wordplay.
'unusually' is an anagram indicator.
'is'+'a'+'glue'+'not'='isagluenot'
'isagluenot' anagrammed gives 'GELATINOUS'.
(Other definitions for gelatinous that I've seen before include "Of a jelly-like consistency" , "description of size" .)