John ducks under end of rail
I believe the answer is:
loo
'john' is the definition.
(loo is a kind of john)
'ducks under end of rail' is the wordplay.
'ducks' becomes 'oo' (I've seen this in other clues).
'under' says to put letters next to each other.
'end of' says to take the final letters.
The final letter of 'rail' is 'l'.
'oo' after 'l' is 'LOO'.
(Other definitions for loo that I've seen before include "A colloquial toilet" , "WC - old card game" , "Card-game" , "Ladies, perhaps" , "Card game; the 'smallest room'" .)