Sailor Bill opens two consecutive letters (4)
I believe the answer is:
'sailor' is the definition.
(informal term for a sailor)
'bill opens two consecutive letters' is the wordplay.
'bill' becomes 'ac' (short for account).
'opens' means one lot of letters goes inside another (one set of letters makes an 'opening' in another).
'two consecutive letters' becomes 'JK' (consecutive letters in the alphabet).
'ac' going inside 'jk' is 'JACK'.
(Other definitions for jack that I've seen before include "Tunic for a soldier", "Jill's companion", "-- Sprat; -- Horner", "Man in navy", "Card; man's name".)