Back the last nag leaving the start (7)
I believe the answer is:
endorse
'back' is the definition.
(back can mean to support or endorse)
'last nag leaving the start' is the wordplay.
I cannot quite see how this works, but
'last' could be 'end' (I've seen this in another clue) and 'end' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'the' is the link.
Can you help me to learn more?
(Other definitions for endorse that I've seen before include "Ratify" , "Give one's sanction to" , "Pass" , "Put one's name to, approve" , "Rubber-stamp" .)