Clue: Long for the longer sleepers perhaps half of LNER and one third of LMS used (4-4)
Answer: KING-SIZE
Explanation: Definition: Long for the longer sleepers
Perhaps (to quite the clue) a charade of KING ('half of LNER' could be ER – for *Edwardus Rex*, obviously, not *Elizabeth Regina*) plus SIZE ('one third of LMS' could be S, an abbreviation which is plausible even though I have not been able to track down a justification for it). The definition refers to the bed size. On the surface, LNER (London and North Eastern Railway) and LMS (London Midland ans Scottish) were railway companies prior to nationalisation in 1948