Crashing exciting car across middle of mound is very painful (12)
I believe the answer is:
excruciating
'very painful' is the definition.
(I've seen this before)
'crashing exciting car across middle of mound' is the wordplay.
'crashing' indicates anagramming the letters.
'across' indicates putting letters inside.
'middle of' says to take the centre.
The centre of 'mound' is 'u'.
'exciting'+'car'='excitingcar'
'excitingcar' with letters rearranged gives 'excrciating'.
'excrciating' enclosing 'u' is 'EXCRUCIATING'.
'is' acts as a link.
(Other definitions for excruciating that I've seen before include "Intensely painful" , "Unbearable" .)