Clue: Potato I coated in chocolate and seed put before Queen (5,5)
Answer: MARIS PIPER
Explanation: Definition: Potato
An envelope ('coated in') of 'I' in MARS (Bar, 'chocolate' – is it OK to imply the bar?) plus PIP ('seed') plus ('put before') ER (*Elizabeth Regina*, 'Queen'). Apparently, it is now the most widely grown in the UK, being versatile and resistant to some nematodes.