Encountered going north, work up speed (5)
I believe the answer is:
tempo
'speed' is the definition.
(I've seen this before)
'encountered going north work up' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'going north' says the letters should be written backwards.
'work' becomes 'op' (abbreviation for opus).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'met' back-to-front is 'tem'.
'op' reversed gives 'po'.
'tem'+'po'='TEMPO'
(Other definitions for tempo that I've seen before include "(Musically) the speed to be played" , "Rhythm or speed as in music" , "Beat (music)" , "Musical time" , "Time (music)" .)