Go fast if short of time (3)
I believe the answer is:
die
'go' is the definition.
(I know that go can be written as die)
'fast if short of time' is the wordplay.
'fast' becomes 'diet' (dieting is a kind of fasting).
'if short of' is a deletion indicator.
'time' becomes 't' (abbreviation).
'diet' with 't' taken away is 'DIE'.
(Other definitions for die that I've seen before include "a block?" , "THIRD PART OF 24" , "Stamping device" , "Give out" , "On stage, suffer" .)