Understand I must be in before noon train (4,3)
I believe the answer is:
take aim
'train' is the definition.
(I know that train can be written as take aim)
'understand i must be in before noon' is the wordplay.
'understand' becomes 'take' (I can't explain this - if you can you should give a lot more credence to this answer).
'i must be' becomes 'i' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'before noon' becomes 'am' (as in AM on a clock).
'i' put inside 'am' is 'aim'.
'take'+'aim'='TAKE AIM'
Can you help me to learn more?
(Other definitions for take aim that I've seen before include "Point gun at one's target" , "Line up weapon" , "Point a weapon at the target" , "Sight a target" , "Penultimate order for those shooting" .)