What was in the barn he set fire to? (6)
I believe the answer is:
heifer
'what was in the barn he set fire to?' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'he set fire to?' is the wordplay.
I cannot really see how this works, but
'he' is present in the answer.
an anagram of 'fire' is 'ifer' which is located 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 heifer that I've seen before include "a little lower" , "part of stock" , "Female bovine that has not given birth" , "Farm animal" , "Young cow which has not yet given birth" .)