Imagine avoiding one tricky puzzle! (6)
I believe the answer is:
enigma
'puzzle' is the definition.
(I've seen this in another clue)
'imagine avoiding one tricky' is the wordplay.
'avoiding' is a deletion indicator.
'one' becomes 'i' (Roman numeral).
'tricky' is an anagram indicator.
'imagine' with 'i' taken out is 'magine'.
'magine' is an anagram of 'ENIGMA'.
(Other definitions for enigma that I've seen before include "Difficult problem" , "'Mystery, riddle (6)'" , "Puzzle for gaming" , "Puzzle found in gem, Ian" , "closed book" .)