Lag pipes faultily — failure to meet target (8)
I believe the answer is:
slippage
'failure to meet target' is the definition.
(slipping from a standard)
'lag pipes faultily' is the wordplay.
'faultily' indicates an anagram (faulty version of the letters).
'lag'+'pipes'='lagpipes'
'lagpipes' is an anagram of 'SLIPPAGE'.
(Other definitions for slippage that I've seen before include "Failure to reach a set target" , "Failure to meet target" , "Lag pipes (anag)" , "sliding" , "Lips gape over loss of motion or power" .)