Long for the longer sleepers perhaps half of LNER and one third of LMS used (4-4)
I believe the answer is:
king-size
'long for the longer sleepers perhaps' is the definition.
(large bed size, used by tall people perhaps)
'half of lner and one third of lms used' is the wordplay.
I cannot really understand how this works, but
'and' could be 'n' (common abbreviation for 'and') and 'n' is found in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
'third of lms' could be 's' (3rd letter of 'lms') and 's' is located in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for king-size that I've seen before include "Extra large, bed perhaps" , "Larger than normal" , "Extra large as for monarch" .)