Writer's up on travel game (5)
I believe the answer is:
bingo
'game' is the definition.
(bingo is a kind of game)
'writer's up on travel' is the wordplay.
'writer' becomes 'nib' (something which writes).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'on' says to put letters next to each other.
'travel' becomes 'go' ('go' can be a synonym of 'travel'**).
'nib' backwards is 'bin'.
'bin'+'go'='BINGO'
(Other definitions for bingo that I've seen before include "Housey-housey" , "Cry on discovering something" , "Game of chance" , "I've completed my card!" , "Game in which "two little ducks" might be called" .)