Knock back second dram and vegetable (7)
I believe the answer is:
parsnip
'vegetable' is the definition.
(parsnip is a type of vegetable)
'knock back second dram' is the wordplay.
'knock' becomes 'rap' (synonyms).
'back' says the letters should be written backwards.
'second' becomes 's' (common abbreviation).
'dram' becomes 'nip' (I've seen this in another clue).
'rap' back-to-front is 'par'.
'par'+'s'+'nip'='PARSNIP'
'and' acts as a link.
(Other definitions for parsnip that I've seen before include "A white root vegetable" , "Tapering vegetable" , "Type of root veg" , "Spar pin (anag) - vegetable" , "Pips ran oddly through the vegetable" .)