Substitute borders on identical seed crop (6)
I believe the answer is:
sesame
'seed crop' is the definition.
'sesame' can be an answer for 'seed' (I've seen this before). I am unsure of the 'crop' bit.
'substitute borders on identical' is the wordplay.
'borders' means to remove the middle letters (only the edge or border letters remain).
'on' means one lot of letters go next to another.
'identical' becomes 'same' (similar in meaning).
'substitute' with its centre taken out is 'se'.
'se'+'same'='SESAME'
Can you help me to learn more?
(Other definitions for sesame that I've seen before include "said to be effective in opening spell" , "Ali Baba's opening command" , "Seed or oil used in cooking" , "Open ...... is magic phrase for opening door in pantomime" , "Source of seed and oil" .)