Fair part of a train wrecked going round a loop line (6)
I believe the answer is:
ration
'fair part' is the definition.
'ration' can be an answer for 'part' (thesaurus). I am not certain of the 'fair' bit.
'train wrecked going round a loop line' is the wordplay.
'wrecked' is an anagram indicator.
'going round' indicates putting letters inside (some letters go around others).
'a loop line' becomes 'o' (I can't justify this - if you can you should give a lot more credence to this answer).
'train' is an anagram of 'ratin'.
'ratin' enclosing 'o' is 'RATION'.
'of a' acts as a link.
Can you help me to learn more?
(Other definitions for ration that I've seen before include "Share of food, etc" , "Limit supply of (eg, food)" , "Limit quantity as of food during scarcity" , "Apportion" , "Dole out portion due to shortage" .)