Speed reduced in desert (3)
I believe the answer is:
rat
'desert' is the definition.
(ratting is a kind of deserting)
'speed reduced' is the wordplay.
'speed' becomes 'rate' (rate is a kind of speed).
'reduced' means to remove the last letter.
'rate' with its last letter taken off is 'RAT'.
'in' acts as a link.
(Other definitions for rat that I've seen before include "Renegade" , "Long-tailed rodent" , "Plague-carrying rodent" , "Inform" , "one who deserts the sinking ship?" .)