Transfer unit back (6)
I believe the answer is:
second
I believe this is a triple definition!
'transfer' is the first definition.
(I've seen this before)
'unit' is the second definition.
(second is a kind of unit)
'back' is the third definition.
(synonyms)
(Other definitions for second that I've seen before include "Brief moment" , "Support; a short time" , "Endorse, back" , "Position of runner-up" , "Small fraction of a day" .)