Keep going at the back of the field (4)
I believe the answer is:
last
'keep going' is the definition.
(I've seen this before)
'at the back of the field' is the wordplay.
'at' says to put letters next to each other.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'back of' shows that the letters should be reversed in order (I've seen 'backed' mean this).
'the field' becomes 'sal' (I am not sure about this - if you are sure you should believe this answer much more).
'sal' backwards is 'las'.
't' after 'las' is 'LAST'.
Can you help me to learn more?
(Other definitions for last that I've seen before include "part of Oxford's restoration with this?" , "Final; endure" , "Hold out" , "Rearmost" , "End" .)