Fertiliser in plant container has worked (6)
I believe the answer is:
potash
'fertiliser' is the definition.
(I've seen this before)
'plant container has worked' is the wordplay.
'plant container' becomes 'pot' (I've seen this before).
'worked' is an anagram indicator.
'has' is an anagram of 'ash'.
'pot'+'ash'='POTASH'
'in' is the link.
(Other definitions for potash that I've seen before include "field dressing" , "Pathos (anag)" , "Potassium compound used as fertiliser" , "Powerful alkali used in gardening" , "Powerful alkali from burnt wood" .)