Don't change this without set-back during work -- hurry (4,2,2)
I believe the answer is:
step on it
'hurry' is the definition.
(I've seen this in another clue)
'don't change this without set-back during work' is the wordplay.
'don't change this' becomes 'stet' (instruction used in publishing).
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'set back' shows that the letters should be reversed in order.
'during' becomes 'in' (e.g. during February means in February).
'work' becomes 'op' (abbreviation for opus).
'in'+'op'='inop'
'inop' reversed gives 'poni'.
'stet' enclosing 'poni' is 'STEP ON IT'.
(Other definitions for step on it that I've seen before include "Hasten" , "be quicker" , "Go for speed" , "Move quickly" , "Informally, hurry or drive faster" .)