Substitute neither first nor last at first (4)
I believe the answer is:
erst
'at first' is the definition.
(I've seen this before)
'substitute neither first nor last' is the wordplay.
I cannot quite see how this works, but
'last' could be 't' (I've seen this in other clues) and 't' is located in the answer.
The remaining letters 'ers' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for erst that I've seen before include "in former times" , "Rest turned formerly" , "in the past" , "At first; formerly" .)