Experimental variables left out, causing friction (8)
I believe the answer is:
abrasive
'friction' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'experimental variables left out' is the wordplay.
'experimental' is an anagram indicator.
'left' becomes 'l' (common abbreviation).
'out' is a deletion indicator.
'variables' with 'l' taken out is 'variabes'.
'variabes' with letters rearranged gives 'ABRASIVE'.
'causing' is the link.
Can you help me to learn more?
(Other definitions for abrasive that I've seen before include "rubbing" , "Rough, grating" , "Grinding substance" , "Harsh; eg, emery" , "Rough and scratchy" .)