Pair in slow train travelling west (3)
I believe the answer is:
two
'pair' is the definition.
(a pair means two of something)
'in slow train travelling west' is the wordplay.
'in' indicates a hidden word.
'travelling west' says the letters should be written backwards.
'owt' is hidden in the letters of 'slow train'.
'owt' in reverse letter order is 'TWO'.
(Other definitions for two that I've seen before include "Minimal number of partners" , "Fifth root of thirty-two" , "eg Twins" , "Pair; hour after lunch" , "Even number" .)