Mostly stumble through unimportant details (6)
I believe the answer is:
trivia
'unimportant details' is the definition.
(I've seen this before)
'mostly stumble through' is the wordplay.
'mostly' means to remove the last letter.
'stumble' becomes 'trip' ('trip' can be a synonym of 'stumble').
'through' becomes 'via' (I've seen this before).
'trip' with its final letter taken away is 'tri'.
'tri'+'via'='TRIVIA'
(Other definitions for trivia that I've seen before include "Minor details" , "Matters very little" , "Unimportant things, minor matters" , "useless bits of information?" , "Non-essential data" .)