Network infiltrated by New Labour (5)
I believe the answer is:
grind
'labour' is the definition.
(I know that grind is a type of labour)
'network infiltrated by new' is the wordplay.
'network' becomes 'grid' (grid is a kind of network).
'infiltrated by' indicates putting letters inside.
'new' becomes 'n' (abbreviation as in New Testament NT).
'grid' enclosing 'n' is 'GRIND'.
(Other definitions for grind that I've seen before include "Jar" , "drudgery" , "Crush into particles" , "hard slog" , "Pulverize" .)