Rock fans nod assent (9)
I believe the answer is:
sandstone
'rock' is the definition.
(sandstone is a kind of rock)
'fans nod assent' is the wordplay.
'fans' is an anagram indicator (I've seen 'fan' mean this (letters must be fanned around)).
'nod'+'assent'='nodassent'
'nodassent' is an anagram of 'SANDSTONE'.
(Other definitions for sandstone that I've seen before include "Such building material" , "Rock containing quartz grains" , "Among the best of building materials" , "Type of sedimentary rock" .)