Work encountered over time (5)
I believe the answer is:
tempo
'time' is the definition.
(tempo is a kind of time)
'work encountered over' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'over' shows that the letters should be reversed in order.
'op'+'met'='opmet'
'opmet' reversed gives 'TEMPO'.
(Other definitions for tempo that I've seen before include "Musical beat or rhythm" , "Speed at which piece of music is played" , "Pace of a piece of music" , "Rhythm of piece of music" , "Musical time" .)