Confusion of woody grass unknown element topped (13)
I believe the answer is:
bamboozlement
'confusion' is the definition.
(I've seen this before)
'woody grass unknown element topped' is the wordplay.
'woody grass' becomes 'bamboo' (I've seen this before).
'unknown' becomes 'z' (represents an unknown value in algebra).
'topped' means to remove the first letter (to top something is to remove its top).
'element' with its initial letter taken off is 'lement'.
'bamboo'+'z'+'lement'='BAMBOOZLEMENT'
'of' is the link.
(Another definition for bamboozlement that I've seen is " confusion".)