Genetic code - with two lines describing the old from the east Lancashire town (7)

'lancashire town' is the definition.
(town in Lancashire)

'genetic code with two lines describing the old from the east' is the wordplay.
'genetic code' becomes 'dna' (DNA is genetic material).
'with two lines' becomes 'll' (I can't explain this - if you can you should give a lot more credence to this answer).
'describing' indicates putting letters inside (describe can mean to form the outline of).
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'from the east' says the letters should be written in reverse.
'dna'+'ll'='dnall'
'dnall' going around 'ye' is 'dnalyel'.
'dnalyel' back-to-front is 'LEYLAND'.