Replace gear thief lifted (6)
I believe the answer is:
retool
'replace gear' is the definition.
'retool' can be an answer for 'replace' (retooling is a kind of replacing). I'm unsure of the 'gear' bit.
'thief lifted' is the wordplay.
'thief' becomes 'looter' (looter is a kind of thief).
'lifted' shows that the letters should be reversed in order (going up in down clue).
'looter' reversed gives 'RETOOL'.
Can you help me to learn more?
(Another definition for retool that I've seen is " Rearrange equipment".)