I am back! Left it to raise top class army reserves (7)
I believe the answer is:
militia
'army reserves' is the definition.
'militia' can be an answer for 'reserves' (synonyms). I'm unsure of the 'army' bit.
'i am back left it to raise top class' is the wordplay.
'i am' becomes 'i'm'.
'back' says the letters should be written in reverse.
'left' becomes 'l' (common abbreviation).
'to raise' shows that the letters should be reversed in order.
'top class' becomes 'ai' (I've seen this in other clues).
'im' reversed gives 'mi'.
'ai' back-to-front is 'ia'.
'mi'+'l'+'it'+'ia'='MILITIA'
Can you help me to learn more?
(Other definitions for militia that I've seen before include "Armed civilian force" , "special reserve" , "Armed civilians" , "Home Guard" , "Civilian soldiery" .)