New bug caught by team going up stream (6)
I believe the answer is:
influx
'stream' is the definition.
(I've seen this before)
'new bug caught by team going up' is the wordplay.
'new' becomes 'n'.
'bug' becomes 'flu'.
'caught by' indicates putting letters inside.
'team' becomes 'xi' (eleven, a football team).
'going up' shows that the letters should be reversed in order.
'xi' written backwards gives 'ix'.
'n'+'flu'='nflu'
'nflu' going inside 'ix' is 'INFLUX'.
(Other definitions for influx that I've seen before include "Arrival of a large number of people / things" , "Flow of people into a place" , "Continual stream of people" , "Large-scale arrival" , "Arrival - invasion" .)