Three hundred on assorted treks cheat (9)
I believe the answer is:
trickster
'cheat' is the definition.
(I know that cheat can be written as trickster)
'three hundred on assorted treks' is the wordplay.
'three' becomes 'tri' (prefix meaning 'three').
'hundred' becomes 'c' (Roman numeral).
'on' means one lot of letters goes inside another.
'assorted' is an anagram indicator.
'treks' is an anagram of 'kster'.
'c' placed inside 'kster' is 'ckster'.
'tri'+'ckster'='TRICKSTER'
(Other definitions for trickster that I've seen before include "Cheat, deceiver" , "Loki, say" , "Hoaxer" , "Practical joker" , "Conman" .)