Help up and along train crossing (8)
I believe the answer is:
diagonal
'crossing' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I can't see?
'help up and along train' is the wordplay.
'help' becomes 'aid' (synonyms).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'and' means one lot of letters go next to another.
'train' indicates an anagram (I've seen 'trained' mean this (to train can mean to grow something into a shape)).
'aid' backwards is 'dia'.
'along' anagrammed gives 'gonal'.
'dia'+'gonal'='DIAGONAL'
Can you help me to learn more?
(Other definitions for diagonal that I've seen before include "Slanting line joining two opposite corners" , "Corner-to-corner line" , "Slanting, from corner to corner" , "A straight line joining two opposite corners" , "Straight line on a slant" .)