Take a look when reversing and hold back (4)
I believe the answer is:
keep
'hold back' is the definition.
(I know that hold back can be written as keep)
'take a look when reversing' is the wordplay.
'take a look' becomes 'peek' (to peek at something is to look at it quickly).
'when reversing' is a reversal indicator.
'peek' backwards is 'KEEP'.
'and' is the link.
(Other definitions for keep that I've seen before include "Retain in old castle tower?" , "Eg, beef" , "Castle tower; cost of maintenance" , "Observe - reserve - preserve" , "Board and lodging" .)