Competition plan loses backing a bit late in the day (8)
I believe the answer is:
eventide
'late in the day' is the definition.
(eventide means the evening)
'competition plan' is the wordplay.
'competition' becomes 'event' (I've seen this before).
'plan' becomes 'ide' (I can't justify this - if you can you should believe this answer much more).
'event'+'ide'='EVENTIDE'
'loses backing a bit' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for eventide that I've seen before include "latter part of the day" , "End of the day (poetic)" , "End of the day to the poet" , "when it starts to get dark" , "onset of dusk" .)