I'd enter uphill races beside the motorcycle (7)
I believe the answer is:
sidecar
'beside the motorcycle' is the definition.
I can't judge whether this definition defines the answer.
'i'd enter uphill races' is the wordplay.
'enter' is an insertion indicator.
'uphill' is a reversal indicator (in a down clue, letters go up).
'races' reversed gives 'secar'.
'id' placed inside 'secar' is 'SIDECAR'.
Can you help me to learn more?
(Other definitions for sidecar that I've seen before include "Passenger carrier attached to a motorcycle" , "that biker may take with him" , "passenger compartment" , "Part of combination" , "Appendage to motor cycle" .)