Wandering, one falls into terrible crater (7)
I believe the answer is:
erratic
'wandering' is the definition.
(I know that wandering can be written as erratic)
'one falls into terrible crater' is the wordplay.
'one' becomes 'i' (Roman numeral).
'falls into' indicates putting letters inside.
'terrible' indicates an anagram.
'crater' anagrammed gives 'erratc'.
'i' going into 'erratc' is 'ERRATIC'.
(Other definitions for erratic that I've seen before include "Mercurial" , "Irregular in pattern or movement" , "Haphazard" , "Irregular and unpredictable" , "hit-and-miss" .)