Most extensive ten logs to be chopped (7)
I believe the answer is:
longest
'most extensive' is the definition.
(I've seen this in another clue)
'ten logs to be chopped' is the wordplay.
'to be chopped' is an anagram indicator (letters chopped or smashed up).
'ten'+'logs'='tenlogs'
'tenlogs' is an anagram of 'LONGEST'.
(Other definitions for longest that I've seen before include "Extending farthest" , "Least short" , "For the most time" , "Having the most length" , "Of greatest duration" .)