Bag taken to Lech on Saturday by mistake. (7)
I believe the answer is:
satchel
'bag' is the definition.
(satchel is a type of bag)
'taken to lech on saturday by mistake' is the wordplay.
I cannot quite understand how this works, but
'taken' could be 'sat' (sit can mean to take an exam) and 'sat' is found in the answer.
an anagram of 'lech' is 'chel' which is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for satchel that I've seen before include "Shoulder bag as for schoolbooks" , "Get school item" , "What's carried by pupils" , "Bag for school books" , "Carrier for school books" .)