Flow of runs in second eleven, perhaps (6)
I believe the answer is:
stream
'flow' is the definition.
(stream is a kind of flow)
'runs in second eleven perhaps' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'in' means one lot of letters goes inside another.
'second' becomes 's' (common abbreviation).
'eleven perhaps' becomes 'team' (eleven is a kind of team).
's'+'team'='steam'
'r' put within 'steam' is 'STREAM'.
'of' acts as a link.
(Other definitions for stream that I've seen before include "Brook, burn" , "comprising a load of drips?" , "'Brook, runnel (6)'" , "selected pupils" , "Small narrow flow" .)