Held up the car reversing into a garden plot (6)
I believe the answer is:
braced
'held up' is the definition.
(bracing is a kind of holding up)
'the car reversing into a garden plot' is the wordplay.
'reversing' shows that the letters should be reversed in order.
'into' indicates putting letters inside.
'a garden plot' becomes 'bed' (I've seen this before).
'car' in reverse letter order is 'rac'.
'rac' inserted within 'bed' is 'BRACED'.
(Other definitions for braced that I've seen before include "Stiff" , "Ready for action" , "Steadied against a blow or shock" , "Supported, gave firmness to" , "Buttressed" .)