Luck, getting in at rising comprehensive (5)
I believe the answer is:
total
'comprehensive' is the definition.
(I know that comprehensive can be written as total)
'luck getting in at rising' is the wordplay.
'luck' becomes 'lot' (synonyms).
'getting in' indicates putting letters inside.
'rising' says the letters should be written in reverse (in a down clue, letters go up).
'lot' placed around 'at' is 'latot'.
'latot' written backwards gives 'TOTAL'.
(Other definitions for total that I've seen before include "Unqualified" , "to give the final figure" , "Aggregate amount" , "It might be grand" , "energy provider" .)