Top-up has artificial fertiliser (6)
I believe the answer is:
potash
'fertiliser' is the definition.
(I know that potash is a type of fertiliser)
'top-up has artificial' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'top' is 'pot' which is within the answer.
an anagram of 'has' is 'ash' which is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for potash that I've seen before include "Alkali used as fertiliser" , "Powerful alkali from burnt wood" , "Pathos (anag)" , "Powerful alkali used in gardening" , "Potassium compound used as fertiliser" .)